lanework/Kanban/LiveStore/BoardRegistry.swift

import Foundation
import os

// MARK: - Timestamps

/// The one shape a `Date` takes in the registry file — ISO 8601 **with fractional seconds**, UTC.
///
/// **Why fractional seconds** rather than `JSONEncoder.DateEncodingStrategy.iso8601`'s whole ones:
/// `lastOpened` is not a display stamp, it is the recents list's *sort key* (02-architecture.md
/// § Per-board app state, "The recents list *is* this registry sorted by last-opened"). Restoring
/// several boards at launch happens well inside one second, and at whole-second resolution those
/// rows would tie and order arbitrarily.
///
/// **Why `ISO8601DateFormatter`** and not `Date.ISO8601FormatStyle`, the modern value type that
/// would be `Sendable` and need none of the ceremony below: the format style *truncates* to the
/// millisecond while its own parser rounds, so `parse(format(d)) != d` for about half of all dates
/// (measured, not assumed — it drifts a millisecond earlier on each round trip). This formatter is
/// an exact fixed point, which is what lets `BoardRegistry.stamp(_:)` guarantee that a record in
/// memory and the same record reloaded from disk are *equal*, not merely close.
///
/// **Built per use rather than shared**: the formatter is a non-`Sendable` class and the encoder's
/// and decoder's date strategies are `@Sendable` closures, so a captured or global instance would
/// be an unchecked concurrency promise. One costs microseconds and this file holds a handful of
/// records — the safe answer is also the cheap one.
private func boardRegistryTimestampFormatter() -> ISO8601DateFormatter {
    let formatter = ISO8601DateFormatter()
    formatter.formatOptions = [.withInternetDateTime, .withFractionalSeconds]
    formatter.timeZone = TimeZone(identifier: "UTC")
    return formatter
}

// MARK: - Records

/// A board's window frame, remembered per board (02-architecture.md § Windows, "per-board frame
/// memory").
///
/// Stored as four `Double`s rather than an `NSRect`/`CGRect` so the persisted JSON is a shape this
/// file owns, independent of any framework's `Codable` conformance. Repositioning a frame onto a
/// live screen when the saved one is gone is the window layer's job, not this type's — nothing here
/// validates the numbers.
public struct WindowFrame: Codable, Sendable, Equatable {
    public var x: Double
    public var y: Double
    public var width: Double
    public var height: Double

    public init(x: Double, y: Double, width: Double, height: Double) {
        self.x = x
        self.y = y
        self.width = width
        self.height = height
    }
}

/// One known board: everything the app keeps *about* a board that must never be written *into* it.
///
/// **Files-first is absolute** (02-architecture.md § Per-board app state): no frontmatter key, no
/// sidecar, no xattr — this record lives wholly in the shared App Group container, which is also why
/// two machines sharing a board through a remote each keep their own (push-on-commit and window
/// frames are genuinely per-machine choices).
///
/// The **bookmark is the identity**; `lastKnownPath` is display text and nothing else. Matching an
/// opened folder to its record resolves the bookmark and compares file identity, so a board renamed
/// or moved on the same volume keeps its settings and its place in recents.
///
/// ### One record, shared by every edition — with two per-edition fields
///
/// The record lives in the family App Group container (`AppGroup`), so base, Pro and later Teams all
/// read and write the same one: an upgrader's recents, frames and settings are simply *there*
/// (12-editions.md ▸ Distribution, ruled 2026-07-29). Two fields cannot be common, and both are
/// keyed by bundle id for the same reason:
///
/// - **`grants`** — security-scoped bookmarks never cross sandboxes (12: "minted per sandbox, App
///   Group or not"), so each edition holds its own. A record whose only grant another edition minted
///   resolves *unavailable-until-reopened* (`RecentBoard.needsReopen`) and its first click runs an
///   open panel pre-anchored at `lastKnownPath`.
/// - **`openNow`** — "an edition restores only the boards *it* had open" (12; 02 § Launch and window
///   lifecycle), which is also what lets the board popover say "Also open in Lanework Pro" from the
///   *other* edition's flag.
///
/// Everything else here is common, deliberately: a window frame, a cached title, a lane count and a
/// push-on-commit choice are facts about the board and this machine, not about which app is looking.
///
/// ### Evolving this struct
///
/// `BoardRegistry` responds to a file it cannot decode by quarantining it — every record lost. So
/// **a key added here must be optional or have a decoding default**, or the addition silently
/// empties every existing user's recents on upgrade. That policy, not a version field, is what keeps
/// this format readable across releases; `init(from:)` below applies it by hand for the keys that
/// arrived with the App Group, and keeps the four founding keys required exactly as they were.
public struct BoardRecord: Codable, Sendable, Equatable, Identifiable {
    public let id: UUID

    /// The per-edition security-scoped grant slots — **the board's identity, per sandbox** — keyed by
    /// bundle id and refreshed on every open *by that edition*. Empty for an edition means "this app
    /// has never been granted this board", which is the unavailable-until-reopened state and not an
    /// error.
    ///
    /// A record with no grants at all is the degenerate case where the system refused to mint one:
    /// born orphaned, shown in recents with Forget, never matching an open.
    public var grants: [String: Data]

    /// Per-edition open-now flags, keyed by bundle id — see `isOpen(inEdition:)` for what the flag
    /// *means*, which is unchanged; only its keying is new.
    public var openNow: [String: Bool]

    /// The pre-App-Group single-grant key (`bookmark`), decoded and **never re-encoded**.
    ///
    /// Its whole life is `BoardRegistry.adoptLegacyKeys()`: a record written before the grant slots
    /// existed has one bookmark, minted by whichever edition wrote it — and since only base existed
    /// then, adopting it as the *running* edition's slot is the coherent reading. Adopted in memory
    /// at load and gone from the file on the first save, which is why nothing else here ever looks
    /// at it.
    public private(set) var legacyBookmark: Data?

    /// The pre-App-Group single open-now key (`isOpenNow`), on the same terms as `legacyBookmark`.
    public private(set) var legacyOpenNow: Bool?

    /// Last-known title or folder name, for the recents row. Display only.
    public var displayName: String

    /// Where the board was last seen, for the recents row when the bookmark no longer resolves and
    /// there is nothing else to show.
    ///
    /// **Not how a board is identified** — that is the grant slot's job, and a path used as the primary
    /// key would reintroduce exactly the identity-by-string bug this design excludes. It has precisely
    /// one narrow other use, which 12-editions.md itself nominates: for a record only *another* edition
    /// holds a grant for, this is the open panel's anchor and the only locator this app has, so
    /// `BoardRegistry.indexOfRecord(matching:)` falls back to it — after identity has failed, and only
    /// against records holding no grant of ours. Without that, granting a board in the second edition
    /// would fork the shared record in two.
    public var lastKnownPath: String

    public var lastOpened: Date

    /// Counts stamped at last close, `nil` until a close has stamped them.
    ///
    /// Registry-cached on purpose: the welcome window renders these instead of scanning
    /// (§ Per-board app state, "no directory scan at welcome time" — which would be slow on a big
    /// board and would hang on an unavailable one). Staleness until the next open is accepted, and a
    /// board that has never been closed simply shows no counts.
    public var laneCount: Int?
    public var cardCount: Int?

    public var windowFrame: WindowFrame?

    /// This board's **card** window frames, keyed by card GUID (05-card-window.md ▸ Window: "frames
    /// restore per card across relaunch where state restoration allows").
    ///
    /// Here rather than in `UserDefaults` for the reason the board's own frame is here: a frame
    /// belongs to a board that the bookmark follows through renames and moves, and a path-keyed
    /// preference would lose every card frame the first time the board was renamed. And here rather
    /// than in the board folder because files-first is absolute — a window frame is per-machine app
    /// state, never board data.
    ///
    /// **The key is the card id case-folded** (`ItemID`'s comparison rule), so a folder respelled
    /// `ABC…` finds the frame stored under `abc…` — the same identity rule the window key itself
    /// uses, since the two must agree about what "this card" means.
    ///
    /// The honest residual: entries accumulate for every card the user has ever opened a window for
    /// on this board, and a card deleted afterwards leaves its entry behind. Accepted — the record
    /// is per-machine convenience state measured in tens of bytes per entry, and it dies wholesale
    /// with Forget like everything else here.
    public var cardWindowFrames: [String: WindowFrame]?

    /// The board's `icon` — registry-cached with live write-through, beside `displayName`
    /// (02-architecture.md § Per-board app state: "The row's title and icon are registry-cached
    /// too — with live write-through"). `nil` when the board's `icon` key is missing or
    /// malformed, which the welcome row reads exactly as the board window itself does: draw the
    /// level default, never a guess (`ItemSymbol`). The value round-trips verbatim as the
    /// frontmatter carries it — an SF Symbol name, unvalidated here; resolving it against what
    /// the running system can draw is the renderer's job, not this record's.
    ///
    /// Stamped at the same moments `displayName` is (`recordOpen`, `recordClose`), and — unlike
    /// the counts — refreshed *live* while the board is open: `BoardRegistry.syncDisplayState`
    /// is the write-through `BoardStore`'s reload pipeline calls into.
    public var icon: String?

    /// The board's `iconColor`, cached beside `icon` for the same reason and at the same
    /// moments. A palette name or a `#RRGGBB[AA]` hex, resolved through `Palette` at render
    /// time — never here.
    public var iconColor: String?

    /// Whether committing also pushes (07-sync-collab.md). Off by default: pushing is a decision,
    /// not a side effect.
    public var pushOnCommit: Bool

    /// Whether this board's once-per-board iCloud/network-volume warning has been shown
    /// (07-sync-collab.md). Once-per-*board*, which is why it lives on the record rather than in
    /// `UserDefaults`.
    public var remoteLocationWarned: Bool

    public init(
        id: UUID = UUID(),
        grants: [String: Data] = [:],
        displayName: String,
        lastKnownPath: String,
        lastOpened: Date,
        laneCount: Int? = nil,
        cardCount: Int? = nil,
        windowFrame: WindowFrame? = nil,
        cardWindowFrames: [String: WindowFrame]? = nil,
        openNow: [String: Bool] = [:],
        pushOnCommit: Bool = false,
        remoteLocationWarned: Bool = false,
        icon: String? = nil,
        iconColor: String? = nil
    ) {
        self.id = id
        self.grants = grants
        self.displayName = displayName
        self.lastKnownPath = lastKnownPath
        self.lastOpened = lastOpened
        self.laneCount = laneCount
        self.cardCount = cardCount
        self.windowFrame = windowFrame
        self.cardWindowFrames = cardWindowFrames
        self.openNow = openNow
        self.pushOnCommit = pushOnCommit
        self.remoteLocationWarned = remoteLocationWarned
        self.icon = icon
        self.iconColor = iconColor
    }

    // MARK: The per-edition slots

    /// This edition's grant, or `nil` when it holds none — the unavailable-until-reopened case.
    ///
    /// An **empty** `Data` answers `nil` too: `recordOpen` stores one when the system refused to mint
    /// a bookmark at all, and a slot holding nothing is indistinguishable from no slot for every
    /// purpose this type has.
    public func grant(forEdition editionID: String) -> Data? {
        guard let grant = grants[editionID], !grant.isEmpty else { return nil }
        return grant
    }

    public mutating func setGrant(_ grant: Data, forEdition editionID: String) {
        grants[editionID] = grant
    }

    /// Whether this board is open in `editionID` right now — the restoration set, as a live marker
    /// rather than an at-quit write (02-architecture.md § Launch and window lifecycle, settled).
    ///
    /// Set when the board's window opens, cleared on *user-initiated* close; quit's teardown
    /// deliberately leaves it standing, because the boards open at quit are by definition the ones
    /// to restore. **Crash recovery falls out for free**: after a crash the flags describe what was
    /// open at crash time, so the next launch restores exactly that — no separate recovery logic, no
    /// once-at-quit stamp to race teardown or miss when the app dies.
    ///
    /// **Per edition** (12-editions.md ▸ Both editions installed): an edition restores only the
    /// boards *it* had open, so a board Pro has open never reopens in base — and the other direction
    /// of the same fact is the popover's awareness line.
    public func isOpen(inEdition editionID: String) -> Bool {
        openNow[editionID] == true
    }

    public mutating func setOpen(_ isOpen: Bool, inEdition editionID: String) {
        openNow[editionID] = isOpen
    }

    /// The editions other than `editionID` that have this board flagged open, sorted so the answer is
    /// stable rather than a dictionary's order.
    ///
    /// **Liveness is not checked here** — that is the caller's, because a flag is a *claim* and a
    /// crashed edition leaves its claims standing by design. See `BoardEditionPresence` for the rule
    /// that turns this list into a line the popover can honestly show.
    public func otherEditionsOpen(besides editionID: String) -> [String] {
        openNow.filter { $0.key != editionID && $0.value }.keys.sorted()
    }

    /// Whether any edition at all holds a grant for this board — what tells
    /// unavailable-until-reopened (some other edition minted the only grant) apart from a genuine
    /// orphan (nobody can reach it).
    public var isGrantedByAnyEdition: Bool {
        grants.contains { !$0.value.isEmpty }
    }

    // MARK: Codable

    /// Hand-written for exactly one reason: the two pre-App-Group keys have to keep decoding while
    /// never being written again (see `legacyBookmark`). Everything else is the synthesized
    /// behaviour restated — required for the four founding keys, so a genuinely broken file still
    /// quarantines, and defaulted for every key added since, which is § Evolving this struct's policy
    /// spelled out rather than implied by an `Optional`.
    private enum CodingKeys: String, CodingKey {
        case id
        case grants
        case openNow
        case displayName
        case lastKnownPath
        case lastOpened
        case laneCount
        case cardCount
        case windowFrame
        case cardWindowFrames
        case icon
        case iconColor
        case pushOnCommit
        case remoteLocationWarned
        /// Pre-App-Group. Read, never written.
        case bookmark
        /// Pre-App-Group. Read, never written.
        case isOpenNow
    }

    public init(from decoder: any Decoder) throws {
        let container = try decoder.container(keyedBy: CodingKeys.self)
        id = try container.decode(UUID.self, forKey: .id)
        displayName = try container.decode(String.self, forKey: .displayName)
        lastKnownPath = try container.decode(String.self, forKey: .lastKnownPath)
        lastOpened = try container.decode(Date.self, forKey: .lastOpened)
        grants = try container.decodeIfPresent([String: Data].self, forKey: .grants) ?? [:]
        openNow = try container.decodeIfPresent([String: Bool].self, forKey: .openNow) ?? [:]
        laneCount = try container.decodeIfPresent(Int.self, forKey: .laneCount)
        cardCount = try container.decodeIfPresent(Int.self, forKey: .cardCount)
        windowFrame = try container.decodeIfPresent(WindowFrame.self, forKey: .windowFrame)
        cardWindowFrames = try container.decodeIfPresent([String: WindowFrame].self, forKey: .cardWindowFrames)
        icon = try container.decodeIfPresent(String.self, forKey: .icon)
        iconColor = try container.decodeIfPresent(String.self, forKey: .iconColor)
        pushOnCommit = try container.decodeIfPresent(Bool.self, forKey: .pushOnCommit) ?? false
        remoteLocationWarned = try container.decodeIfPresent(Bool.self, forKey: .remoteLocationWarned) ?? false
        legacyBookmark = try container.decodeIfPresent(Data.self, forKey: .bookmark)
        legacyOpenNow = try container.decodeIfPresent(Bool.self, forKey: .isOpenNow)
    }

    public func encode(to encoder: any Encoder) throws {
        var container = encoder.container(keyedBy: CodingKeys.self)
        try container.encode(id, forKey: .id)
        try container.encode(grants, forKey: .grants)
        try container.encode(openNow, forKey: .openNow)
        try container.encode(displayName, forKey: .displayName)
        try container.encode(lastKnownPath, forKey: .lastKnownPath)
        try container.encode(lastOpened, forKey: .lastOpened)
        try container.encodeIfPresent(laneCount, forKey: .laneCount)
        try container.encodeIfPresent(cardCount, forKey: .cardCount)
        try container.encodeIfPresent(windowFrame, forKey: .windowFrame)
        try container.encodeIfPresent(cardWindowFrames, forKey: .cardWindowFrames)
        try container.encodeIfPresent(icon, forKey: .icon)
        try container.encodeIfPresent(iconColor, forKey: .iconColor)
        try container.encode(pushOnCommit, forKey: .pushOnCommit)
        try container.encode(remoteLocationWarned, forKey: .remoteLocationWarned)
        // `bookmark` and `isOpenNow` are deliberately absent: this is the tolerate-and-upgrade half
        // of backward compatibility. An unknown key is dropped on the same terms — the synthesized
        // conformance dropped them too, so the file's forward tolerance is exactly what it was.
    }

    /// Folds the two pre-App-Group keys into `editionID`'s slots and forgets them.
    ///
    /// **Only when the modern slot is empty**, in both cases: a file carrying both shapes was written
    /// by a build that already knew about grants, and the legacy key is then stale by definition.
    ///
    /// **In memory only, at load.** The upgraded shape reaches disk on the next ordinary save —
    /// "tolerate-and-upgrade on first write" — so merely *reading* a registry never rewrites it, and
    /// a launch that opens nothing leaves the file exactly as it found it.
    mutating func adoptLegacyKeys(as editionID: String) {
        if let legacyBookmark, !legacyBookmark.isEmpty, grants.isEmpty {
            grants[editionID] = legacyBookmark
        }
        if legacyOpenNow == true, openNow.isEmpty {
            openNow[editionID] = true
        }
        legacyBookmark = nil
        legacyOpenNow = nil
    }
}

/// A recents row: the record, plus whether its board can be reached right now.
///
/// The classification is a **bookmark resolution and nothing else** — no `fileExists`, no listing,
/// no counting. That is what makes `BoardRegistry.recents()` safe to call while a network volume is
/// offline (§ Per-board app state, and § Graceful orphaning for what `unavailable` means to the UI:
/// the row shows with Forget rather than disappearing).
public enum RecentBoard: Sendable, Equatable {
    /// Resolved. `at` is where the board lives *now*, which is not necessarily `lastKnownPath` — a
    /// bookmark follows renames and moves on the same volume.
    case available(BoardRecord, at: URL)
    /// The bookmark no longer resolves: deleted, or moved across a volume boundary a bookmark
    /// cannot follow. Orphaned — "its settings are conveniences and die with it".
    case unavailable(BoardRecord)
    /// **Unavailable-until-reopened**: this edition holds no grant, but another edition does
    /// (12-editions.md ▸ Distribution — "a record another edition minted resolves
    /// unavailable-until-reopened, and the first click runs an open panel pre-anchored at the
    /// recorded path: one click + Grant per board, once per edition").
    ///
    /// `recordedAt` is `lastKnownPath` as a URL — the panel's anchor, and the one place that field is
    /// used for anything but display. It is **not** a claim that the board is there: nothing here
    /// touches the filesystem, and a board that has since moved simply opens the panel at its parent.
    case needsReopen(BoardRecord, recordedAt: URL)

    public var record: BoardRecord {
        switch self {
        case let .available(record, _): record
        case let .unavailable(record): record
        case let .needsReopen(record, _): record
        }
    }

    /// Where the board is now, or `nil` when this edition cannot reach it — an orphan, or a record
    /// awaiting this edition's grant. Both answer `nil` because both need something to happen before
    /// a board can be opened; which something is `regrantAnchor`'s question.
    public var url: URL? {
        switch self {
        case let .available(_, url): url
        case .unavailable, .needsReopen: nil
        }
    }

    /// Where an open panel should start when this row is clicked, or `nil` for a row a panel cannot
    /// help — an available board (nothing to grant) or a genuine orphan (nothing to grant *to*).
    public var regrantAnchor: URL? {
        switch self {
        case let .needsReopen(_, url): url
        case .available, .unavailable: nil
        }
    }
}

// MARK: - BoardRegistry

/// The persistent side of per-board app state: one record per known board, in the shared App Group
/// container (02-architecture.md § Per-board app state; `AppGroup`).
///
/// ### Three rules do most of the work
///
/// 1. **Identity, never paths.** An opened folder finds its record by resolving each record's
///    bookmark and comparing file identity. A renamed board keeps its settings; a board reopened
///    through a symlink or a moved parent does not acquire a second record.
/// 2. **Nothing it does touches a board folder.** Not a byte, not an xattr. The one file it writes
///    is its own, and it lives somewhere else entirely.
/// 3. **It never takes the app down.** A missing file is an empty registry; a corrupt one is moved
///    aside and the registry starts empty; a save that fails is logged. Every method here is
///    non-throwing on purpose — "its settings are conveniences", and losing a window frame must
///    never cost the user a board.
///
/// ### Deliberately not here
///
/// App-wide state that is not board-scoped — quick-style recents, the SSH host-key table, the
/// last-used card-window size — lives *beside* this file, not in it (§ Per-board app state, "App-wide
/// state has the same home"). Secrets live in the Keychain and nowhere near here.
@MainActor
public final class BoardRegistry {

    /// The JSON file this registry is. Public because a diagnostic ("Reveal registry in Finder") and
    /// every test want to name it, and because injecting it is how a test stays out of the real shared
    /// App Group container (`AppGroup`) — which after the 2026-07-29 ruling is the sibling edition's
    /// registry too, so a suite writing there would be editing two apps' state.
    public let storageURL: URL

    /// **Which edition's slots this registry reads and writes** — the bundle id keying `grants` and
    /// `openNow` on every shared record.
    ///
    /// Injected rather than read from `Bundle.main` at each use for the reason `storageURL` is
    /// injected: it is how a test can be Pro looking at base's records (and back again) inside one
    /// process, which is the only way the cross-edition rules are checkable at all.
    public let editionID: String

    private var records: [BoardRecord]

    private static let logger = Logger(subsystem: "dev.rzen.indie.Kanban", category: "board-registry")

    /// `<group container>/Library/Application Support/board-registry.json` — the **shared** home
    /// (12-editions.md ▸ Distribution, ruled 2026-07-29), with no bundle-id subfolder, because the
    /// absence of that subfolder is what makes one list serve every edition.
    ///
    /// `AppGroup.stateDirectory` falls back to the old per-edition path when the group is not
    /// provisioned, so this is also the answer in a test host and in a locally signed build.
    public static var defaultStorageURL: URL {
        AppGroup.stateDirectory.appendingPathComponent("board-registry.json", isDirectory: false)
    }

    /// Loads the registry, tolerating everything a file on disk can be.
    ///
    /// Missing file: an empty registry — the first-launch case, and not an error. **Corrupt file:
    /// renamed aside with a timestamp and the registry starts empty.** Renamed rather than deleted
    /// because the file may be the only trace of a user's board list, and a support request can read
    /// it even when this app cannot; empty rather than fatal because a truncated convenience file
    /// must not stand between the user and their boards.
    ///
    /// A file written before the per-edition slots existed is upgraded **in memory** on the way in
    /// (`BoardRecord.adoptLegacyKeys(as:)`) — its one bookmark becomes this edition's grant — and
    /// reaches disk in the new shape on the next ordinary save.
    public init(storageURL: URL, editionID: String = AppGroup.editionID) {
        self.storageURL = storageURL
        self.editionID = editionID
        self.records = []
        self.records = loadRecords()
        self.fileStamp = Self.fileStamp(of: storageURL)
    }

    // MARK: - Two editions, one file

    /// What the file looked like as of this registry's last read or write.
    ///
    /// The whole of the cross-edition freshness mechanism (see `syncFromDiskIfChanged`). `nil` means
    /// "there is no file", which is the first-launch state and not a stale one.
    private var fileStamp: FileStamp?

    private struct FileStamp: Equatable {
        let modified: Date
        let size: Int
    }

    /// **`FileManager.attributesOfItem` and deliberately not `URL.resourceValues`.** A `URL` *caches*
    /// resource values on the instance it was asked through, and this registry holds one `storageURL`
    /// for its whole life — so the second question would be answered with the first question's answer,
    /// and a stamp that never changes is a freshness check that never fires. (Measured, not assumed:
    /// with `resourceValues` here, each edition kept its own pre-write view and the last save won
    /// wholesale, which is exactly the bug this mechanism exists to prevent.)
    private static func fileStamp(of url: URL) -> FileStamp? {
        guard let attributes = try? FileManager.default.attributesOfItem(atPath: url.path),
              let modified = attributes[.modificationDate] as? Date,
              let size = attributes[.size] as? Int
        else { return nil }
        return FileStamp(modified: modified, size: size)
    }

    /// Re-reads the file when somebody else has written it — **the sibling edition**.
    ///
    /// ### Why this is needed at all
    ///
    /// The registry is one shared file now (12-editions.md ▸ Distribution) and "the same board open in
    /// both apps at once is fine … a supported steady state, not a transition to hurry past" (▸ Both
    /// editions installed). But this type holds its records in memory for the life of a launch and
    /// `save()` writes the array *wholesale* — so without this, base running for an hour would, on its
    /// next window-frame save, silently erase every record Pro wrote in that hour. That is not a narrow
    /// race; it is the ordinary outcome of the steady state the design blesses.
    ///
    /// **Disk is the truth and this cache is only a cache**, which is what makes the fix this small:
    /// there is no in-memory state to reconcile, because every mutation here saves immediately. So a
    /// reload is a plain replacement, and the merge problem never arises.
    ///
    /// ### Why a stamp rather than an unconditional read
    ///
    /// One `stat` instead of a file read and a JSON parse, and — the part that matters — **exactly zero
    /// behavioural change when one edition is running**: the stamp is recorded on every save, so our own
    /// writes never look like somebody else's. Sub-second `contentModificationDate` plus the size makes
    /// a missed change vanishingly unlikely, and the cost of missing one is the pre-existing behaviour.
    ///
    /// The honest residual, stated: the window between this check and the write that follows it is still
    /// last-writer-wins. It is microseconds rather than hours, and what it can cost is one convenience
    /// field — 02 § Per-board app state's "its settings are conveniences" already accepts exactly that.
    private func syncFromDiskIfChanged() {
        let current = Self.fileStamp(of: storageURL)
        guard current != fileStamp else { return }
        Self.logger.debug("the registry file changed underneath us — re-reading the sibling edition's writes")
        fileStamp = current
        records = loadRecords()
    }

    /// The load, plus the legacy-key adoption every load applies (see `init`).
    private func loadRecords() -> [BoardRecord] {
        loadFromDisk().map { record in
            var record = record
            record.adoptLegacyKeys(as: editionID)
            return record
        }
    }

    // MARK: - Opening and closing

    /// Records that a board was opened, and answers with the id of the record it belongs to.
    ///
    /// Matching is the interesting half: every existing record's bookmark is resolved and its file
    /// identity compared with the opened folder's. A hit is *the* record no matter what path either
    /// side is spelled with — the done-when criterion for a renamed board keeping its settings.
    /// Records whose bookmarks no longer resolve are skipped rather than repaired: an orphan is a
    /// recents row with Forget, not a candidate for a board that is demonstrably somewhere else.
    ///
    /// A match is updated in place with a **fresh bookmark** (subsuming the stale-refresh case), the
    /// path it was opened at, and `lastOpened` = now. No match creates a record. Either way the
    /// file is saved before returning.
    ///
    /// **`isOpenNow` is deliberately untouched here.** Recording an open and *being* open are two
    /// different facts: this method is called before a window exists (and, later, by flows that
    /// record a board without showing one), so the flag is set by `setOpenNow(id:)` once the window
    /// has actually opened. Folding it in would flag boards that never made it onto screen and hand
    /// the next launch a restoration set describing failures.
    ///
    /// ### `displayName` is `nil` before a load has run — that is the whole of "record before load"
    ///
    /// 02-architecture.md § Per-board app state (settled): **"the registry record is created before
    /// loading"**, and **"the record's provisional display name is the folder name … the first
    /// successful load replaces it with the cached title."** This method is the one open-time stamp
    /// both moments share, told apart by whether the caller has anything authoritative to say yet:
    ///
    /// - `nil` (the "before load" call, `BoardWindowHost.start()`'s first act): the bookmark,
    ///   `lastKnownPath`, and `lastOpened` refresh as always, but `displayName`/`icon`/`iconColor`
    ///   are left **untouched** on a record that already has them — the last successful open's
    ///   cached title is still the best information this board's row has, and a load that is about
    ///   to fail must not regress it to the bare folder name. A **brand-new** record has nothing
    ///   cached yet, so it takes the folder name — "a never-successfully-opened record is not a
    ///   special class; it lingers in recents like any other."
    /// - Non-`nil` (a caller with real, loaded values): overwrites `displayName`/`icon`/`iconColor`
    ///   unconditionally, exactly as before this distinction existed. Nothing in this app calls it
    ///   that way any more — a successful load's replacement goes through `syncDisplayState`
    ///   instead, which shares its no-op-skip discipline with every reload afterward — but the
    ///   parameter stays meaningful on its own rather than folding into a second method, and every
    ///   existing test naming a concrete string exercises exactly this branch.
    ///
    /// **Why this does not cost a second bookmark mint.** `recordOpen` is called exactly once per
    /// open attempt (the "before load" call), so the one bookmark it mints here is the *whole* of
    /// this open's mint (02 § Per-board app state, "one bookmark per open board"). The successful-load
    /// follow-up is `syncDisplayState`, which never touches `bookmark` at all.
    @discardableResult
    public func recordOpen(
        of rootURL: URL,
        displayName: String? = nil,
        icon: String? = nil,
        iconColor: String? = nil
    ) -> UUID {
        syncFromDiskIfChanged()

        let bookmark = Self.makeBookmark(for: rootURL)?.data ?? Data()
        if bookmark.isEmpty {
            // Both the security-scoped and the plain attempt failed — vanishingly unlikely for a
            // folder the app has just opened. The record is still created: a row the user can see
            // and Forget beats a board that silently never joins recents.
            Self.logger.error("no bookmark could be created for the opened board; its record is born orphaned")
        }

        if let index = indexOfRecord(matching: rootURL) {
            // **This edition's slot only.** A match found through the cross-edition fallback below is
            // precisely the re-grant flow (12-editions.md: "one click + Grant per board, once per
            // edition") — it mints *this* edition's grant onto the shared record and leaves the other
            // edition's untouched, so the board stays reachable from both.
            records[index].setGrant(bookmark, forEdition: editionID)
            if let displayName {
                records[index].displayName = displayName
                records[index].icon = icon
                records[index].iconColor = iconColor
            }
            records[index].lastKnownPath = rootURL.path
            records[index].lastOpened = Self.stamp()
            save()
            return records[index].id
        }

        let record = BoardRecord(
            grants: [editionID: bookmark],
            displayName: displayName ?? Self.folderName(of: rootURL),
            lastKnownPath: rootURL.path,
            lastOpened: Self.stamp(),
            icon: icon,
            iconColor: iconColor
        )
        records.append(record)
        save()
        return record.id
    }

    /// The folder name, extension stripped — `AppModel.folderDisplayName(of:)`'s own rule, restated
    /// here rather than reached for: this file is `Foundation`-only and must not import the app
    /// layer to borrow four words of `URL` math.
    private static func folderName(of url: URL) -> String {
        url.deletingPathExtension().lastPathComponent
    }

    /// Stamps the counts the welcome window will render for this board until it is opened again
    /// (§ Per-board app state, "Recents counts are registry-cached ... stamped at last close").
    ///
    /// Called from the close-flush sequence (02-architecture.md § Launch and window lifecycle),
    /// where the store's last snapshot is still in hand — which is the whole reason the counts are
    /// free here and expensive anywhere else.
    ///
    /// `displayName`/`icon`/`iconColor` are re-stamped here too, from the same last-held snapshot.
    /// Unlike the counts, these three are already kept current while the board is open — every
    /// reload write-throughs via `syncDisplayState` — so this is the belt-and-braces close-time
    /// stamp rather than their only update path: a final, cheap guarantee that closing a board
    /// never leaves its row one edit behind, whatever wired the live path.
    public func recordClose(
        id: UUID,
        displayName: String,
        laneCount: Int,
        cardCount: Int,
        icon: String? = nil,
        iconColor: String? = nil
    ) {
        update(id) { record in
            record.displayName = displayName
            record.icon = icon
            record.iconColor = iconColor
            record.laneCount = laneCount
            record.cardCount = cardCount
        }
    }

    // MARK: - The open-now marker

    /// Marks this board as open — called when its window has actually opened, not when the open was
    /// merely attempted (02-architecture.md § Launch and window lifecycle).
    public func setOpenNow(id: UUID) {
        update(id) { [editionID] in $0.setOpen(true, inEdition: editionID) }
    }

    /// Clears the marker — **user-initiated close only**.
    ///
    /// Quit's teardown must never call this, and that omission is the entire restoration mechanism:
    /// "quit's teardown closes deliberately leave it standing (the boards were open at quit by
    /// definition; teardown distinguishes user-close from quit-close, and that distinction is the
    /// whole mechanism)". A crash calls nothing at all, which is why crash recovery needs no code of
    /// its own — the flags already describe what was open when the app died.
    public func clearOpenNow(id: UUID) {
        update(id) { [editionID] in $0.setOpen(false, inEdition: editionID) }
    }

    /// The **other** editions with this board flagged open — the raw flags, what the board popover's
    /// awareness line is derived from (12-editions.md ▸ Both editions installed: "the board popover
    /// carries a contextual awareness line … read from the other edition's flag,
    /// pid-liveness-checked so crash residue never lies").
    ///
    /// **Deliberately unfiltered.** Liveness is `BoardEditionPresence`'s half and lives there once: a
    /// flag is a claim this file records faithfully, including the stale claim a crashed edition
    /// leaves behind, and deciding which claims are still true needs `NSRunningApplication` — which
    /// this `Foundation`-only file has no business reaching for.
    public func otherEditionsFlaggedOpen(id: UUID) -> [String] {
        record(id: id)?.otherEditionsOpen(besides: editionID) ?? []
    }

    /// The boards to reopen at launch: every record still flagged open, **oldest `lastOpened`
    /// first**.
    ///
    /// Ascending, unlike `recents()`, because these are reopened in order and the result should be
    /// the stacking the user left behind — the most recently opened board ends up frontmost because
    /// it opens last. Classification is `recents()`' own bookmark resolution, reused rather than
    /// re-implemented: a flagged board on an unmounted volume is `unavailable` here for exactly the
    /// reason it is unavailable there, and the launch flow renders it as a failed restoration
    /// (§ "A restored board that fails surfaces on welcome, row-level") instead of hanging on a
    /// mount.
    ///
    /// **This edition's flags only** (12-editions.md ▸ Both editions installed): "an edition restores
    /// only the boards *it* had open". A board Pro has open is Pro's to reopen, and base opening it
    /// too at launch would be the app deciding for the user that two windows onto one board is what
    /// they meant.
    ///
    /// The preference gates only whether this is *consulted*; the flags are maintained regardless.
    public func restorables() -> [RecentBoard] {
        recents()
            .filter { $0.record.isOpen(inEdition: editionID) }
            // Ascending, with the same id tie-break `recents()` uses inverted, so two boards opened
            // in the same millisecond still come back in one stable order rather than whichever
            // `sorted(by:)` felt like.
            .sorted { lhs, rhs in
                lhs.record.lastOpened == rhs.record.lastOpened
                    ? lhs.record.id.uuidString < rhs.record.id.uuidString
                    : lhs.record.lastOpened < rhs.record.lastOpened
            }
    }

    // MARK: - Per-board settings

    public func updateWindowFrame(id: UUID, frame: WindowFrame) {
        update(id) { $0.windowFrame = frame }
    }

    /// One card window's remembered frame on this board, or `nil` when that card has never had one
    /// (05-card-window.md ▸ Window). The caller decides what "no frame" means — for a card window it
    /// means "open at the last-used size and cascade".
    public func cardWindowFrame(id: UUID, cardID: ItemID) -> WindowFrame? {
        record(id: id)?.cardWindowFrames?[cardID.canonicalValue]
    }

    /// Remembers one card window's frame. Keyed by `ItemID`'s own comparison value, so the read
    /// above finds it whatever case the card's folder is spelled in.
    ///
    /// **Unchanged writes nothing**, `syncDisplayState`'s rule: a card window reports its frame on
    /// every move and at the end of every live resize, and the board window's twin already saves on
    /// each of those — this one must not add a registry file write for a frame that did not move.
    public func updateCardWindowFrame(id: UUID, cardID: ItemID, frame: WindowFrame) {
        guard cardWindowFrame(id: id, cardID: cardID) != frame else { return }
        update(id) { record in
            var frames = record.cardWindowFrames ?? [:]
            frames[cardID.canonicalValue] = frame
            record.cardWindowFrames = frames
        }
    }

    /// The live write-through for an *open* board's title, icon, and iconColor
    /// (02-architecture.md § Per-board app state, "these three refresh whenever an open board's
    /// reload changes them"). `BoardStore` calls into this — indirectly, through the delegate
    /// `BoardWindowHost.configureWindow` wires — on every successful reload, so an in-app rename
    /// or restyle lands in the welcome row the instant the store's snapshot shows it, and a
    /// foreign edit of an *open* board's root rides the same reload for free.
    ///
    /// **A no-op, and no save, when nothing differs from what is already cached.** A reload fires
    /// on every tree change anywhere in the board, most of which touch no board-level field at
    /// all, so calling this unconditionally must not churn the registry file on an unrelated card
    /// edit — the same "an unchanged value writes nothing" rule `setLaneWidth` and `commitRename`
    /// already keep.
    ///
    /// An unknown id is `update`'s own no-op (a board closed and forgotten mid-reload), for the
    /// same reason every other setter here tolerates one.
    public func syncDisplayState(id: UUID, title: String, icon: String?, iconColor: String?) {
        syncFromDiskIfChanged()
        guard let index = indexOfRecord(id) else {
            Self.logger.debug("syncDisplayState: no record for this id — ignored")
            return
        }
        guard records[index].displayName != title
            || records[index].icon != icon
            || records[index].iconColor != iconColor
        else { return }

        records[index].displayName = title
        records[index].icon = icon
        records[index].iconColor = iconColor
        save()
    }

    public func setPushOnCommit(id: UUID, _ value: Bool) {
        update(id) { $0.pushOnCommit = value }
    }

    /// Marks this board's once-per-board remote-location warning as shown (07-sync-collab.md).
    /// One-way: there is no un-warn, because the warning is about a location the board is already at.
    public func setRemoteLocationWarned(id: UUID) {
        update(id) { $0.remoteLocationWarned = true }
    }

    // MARK: - Reading

    /// Every known board, most recently opened first, each classified by whether its bookmark
    /// resolves — the recents list, which *is* this registry sorted by last-opened.
    ///
    /// **No directory is scanned and nothing is counted here.** Counts come from the records; the
    /// only filesystem work is bookmark resolution, done with `.withoutUI` and `.withoutMounting`
    /// so an offline volume classifies as unavailable instead of blocking the welcome window on a
    /// mount.
    ///
    /// Two quiet side effects, both of them the "stale is fine — refresh silently" rule: a bookmark
    /// that resolves *stale* is replaced with a fresh one, and if any was, the file is saved. A
    /// stale bookmark still works, but only for a while — refreshing it here is what keeps a board
    /// that moves around from eventually orphaning itself. `lastKnownPath` is deliberately **not**
    /// refreshed: it is the display fallback for a record that *cannot* be resolved, so the resolved
    /// URL, not the record, is what an available row shows.
    ///
    /// **Three states, not two** (12-editions.md ▸ Distribution): a record this edition holds no grant
    /// for, but some other edition does, is `needsReopen` — unavailable-until-reopened, with the
    /// recorded path as the open panel's anchor. A record nobody can reach is the orphan it always
    /// was. Only this edition's slot is ever resolved or refreshed; another edition's bookmark is not
    /// this app's to resolve and would fail if it tried.
    ///
    /// The sort is total — `lastOpened` descending, then id — because `sorted(by:)` is not stable
    /// and two rows that tie should still come back in the same order every call.
    public func recents() -> [RecentBoard] {
        syncFromDiskIfChanged()

        var resolvedURLs: [UUID: URL] = [:]
        var refreshedAny = false

        for index in records.indices {
            guard let grant = records[index].grant(forEdition: editionID),
                  let resolution = Self.resolve(grant) else { continue }
            resolvedURLs[records[index].id] = resolution.url
            guard resolution.isStale else { continue }
            if let refreshed = Self.withScopedAccess(to: resolution.url, { Self.makeBookmark(for: $0) }) {
                records[index].setGrant(refreshed.data, forEdition: editionID)
                refreshedAny = true
            }
        }
        if refreshedAny {
            save()
        }

        return records
            .sorted { lhs, rhs in
                lhs.lastOpened == rhs.lastOpened
                    ? lhs.id.uuidString > rhs.id.uuidString
                    : lhs.lastOpened > rhs.lastOpened
            }
            .map { record in
                if let url = resolvedURLs[record.id] {
                    .available(record, at: url)
                } else if record.grant(forEdition: editionID) == nil, record.isGrantedByAnyEdition {
                    // No grant of ours, but somebody's — the cross-edition row. Note the order: a
                    // grant of ours that *failed to resolve* falls through to `unavailable` below,
                    // which is right. That is Graceful orphaning's case (the board is gone), not the
                    // re-grant case (the board is there and we were never given it).
                    .needsReopen(record, recordedAt: URL(fileURLWithPath: record.lastKnownPath, isDirectory: true))
                } else {
                    .unavailable(record)
                }
            }
    }

    /// One record, fresh — the sibling edition's writes included, which is what lets the popover's
    /// awareness line see a flag Pro set after this app launched.
    public func record(id: UUID) -> BoardRecord? {
        syncFromDiskIfChanged()
        return records.first { $0.id == id }
    }

    /// Drops a record — the Forget action on an orphaned recents row, and the only way a record
    /// leaves. Nothing else prunes: a board that is merely unavailable today may be a remounted
    /// volume tomorrow, so forgetting is always the user's call.
    public func forget(id: UUID) {
        syncFromDiskIfChanged()
        guard let index = indexOfRecord(id) else { return }
        records.remove(at: index)
        save()
    }

    /// Drops every record — File ▸ Open Recent ▸ Clear Menu (11-command-nexus.md).
    ///
    /// **Finder's Clear Menu clears the menu; this registry *is* the menu**, so the equivalence is
    /// exact: there is no separate recents list that could be emptied while the records stayed, and
    /// a record whose board never appears anywhere is a setting nothing can reach. It is therefore
    /// `forget(id:)` applied to every row, and the one test worth writing says exactly that.
    ///
    /// One save rather than one per record: the file is rewritten wholesale anyway, and forgetting
    /// twenty boards should not be twenty writes.
    public func forgetAll() {
        syncFromDiskIfChanged()
        guard !records.isEmpty else { return }
        records.removeAll()
        save()
    }

    // MARK: - Matching

    /// The index of the record whose grant resolves to the same file as `url`, if any — falling back
    /// to the recorded path for records this edition has no grant for at all.
    ///
    /// **File identity is still the rule.** The fallback is not a second identity mechanism; it is the
    /// only locator available for a record only *another* edition can resolve, and it is the same
    /// locator the design already nominates for that case — "an open panel pre-anchored at the
    /// recorded path" (12-editions.md). It is consulted only after identity has failed, and only
    /// against records holding no grant of ours, so a board this edition knows can never be matched by
    /// its path. Without it, granting a board in the second edition would fork the shared record into
    /// two and undo the whole point of sharing it.
    private func indexOfRecord(matching url: URL) -> Int? {
        guard let target = FileIdentity(of: url) else { return nil }

        let byIdentity = records.firstIndex { record in
            guard let grant = record.grant(forEdition: editionID),
                  let resolution = Self.resolve(grant) else { return false }
            // Scope is started around the identity read and stopped immediately. Resolving a
            // security-scoped bookmark grants nothing by itself, and in the sandbox an unscoped
            // `resourceValues` call on some *other* board's folder is exactly the read that gets
            // refused — which would silently turn "the same board" into "a new one" and duplicate
            // the record. The pairing is balanced, so an outer scope this app holds elsewhere is
            // unaffected.
            return Self.withScopedAccess(to: resolution.url) { FileIdentity(of: $0) == target }
        }
        if let byIdentity { return byIdentity }

        let key = Self.pathKey(url.path)
        return records.firstIndex { record in
            record.grant(forEdition: editionID) == nil
                && record.isGrantedByAnyEdition
                && Self.pathKey(record.lastKnownPath) == key
        }
    }

    /// How two paths are compared for "the same board" in the fallback above — standardized, never
    /// resolved through symlinks, which is `WelcomeRow.pathKey`'s rule restated rather than imported
    /// (this file is `Foundation`-only and must not reach into the app layer).
    private static func pathKey(_ path: String) -> String {
        URL(fileURLWithPath: path).standardizedFileURL.path
    }

    private func indexOfRecord(_ id: UUID) -> Int? {
        records.firstIndex { $0.id == id }
    }

    /// Mutates a record and saves. An unknown id is a no-op: a window that outlived its record —
    /// the user pressed Forget while the board was open — must not crash on its way out.
    private func update(_ id: UUID, _ mutate: (inout BoardRecord) -> Void) {
        syncFromDiskIfChanged()
        guard let index = indexOfRecord(id) else {
            Self.logger.debug("update: no record for this id — ignored")
            return
        }
        mutate(&records[index])
        save()
    }

    // MARK: - Bookmarks

    /// A bookmark and which flavor the system actually gave us.
    struct Bookmark {
        let data: Data
        let isSecurityScoped: Bool
    }

    /// Creates a bookmark for `url`, security-scoped if the system allows it.
    ///
    /// **The security-scoped path is the real one.** In production every board URL arrives through
    /// NSOpenPanel, a Finder drag, or a previously resolved bookmark, so the app already holds
    /// access and `.withSecurityScope` succeeds — which is what lets the board reopen after a
    /// relaunch at all (02-architecture.md § Platform, "security-scoped bookmarks for reopening
    /// boards across launches"; the `com.apple.security.files.bookmarks.app-scope` entitlement is
    /// what backs it).
    ///
    /// **The plain fallback exists for the cases where that is not true**: a unit test pointing at a
    /// temp directory the panel never blessed, and a non-sandboxed debug build where the option is
    /// meaningless. A plain bookmark still tracks renames and moves — the identity story is intact —
    /// it simply grants no access on resolution, which outside the sandbox is nothing to grant.
    /// Returning `nil` from both attempts is left to the caller, which records an orphan rather than
    /// dropping the board.
    static func makeBookmark(for url: URL) -> Bookmark? {
        if let data = try? url.bookmarkData(options: [.withSecurityScope]) {
            return Bookmark(data: data, isSecurityScoped: true)
        }
        if let data = try? url.bookmarkData(options: []) {
            Self.logger.debug("security-scoped bookmark unavailable; fell back to a plain bookmark")
            return Bookmark(data: data, isSecurityScoped: false)
        }
        return nil
    }

    struct Resolution {
        let url: URL
        let isStale: Bool
    }

    /// Resolves a bookmark, mirroring `makeBookmark(for:)`'s two flavors in the same order.
    ///
    /// `.withoutUI` and `.withoutMounting`: resolution runs during a recents listing, and a recents
    /// listing must never put up a dialog or block for seconds mounting a server. A board on an
    /// unmounted volume is *unavailable right now*, which is precisely what the row should say.
    ///
    /// Resolution alone does **not** start security-scoped access — the caller does that around the
    /// use, balanced. That is why `recents()` can classify every record without ever holding a scope
    /// open.
    static func resolve(_ bookmark: Data) -> Resolution? {
        guard !bookmark.isEmpty else { return nil }

        var isStale = false
        if let url = try? URL(
            resolvingBookmarkData: bookmark,
            options: [.withSecurityScope, .withoutUI, .withoutMounting],
            relativeTo: nil,
            bookmarkDataIsStale: &isStale
        ) {
            return Resolution(url: url, isStale: isStale)
        }

        isStale = false
        if let url = try? URL(
            resolvingBookmarkData: bookmark,
            options: [.withoutUI, .withoutMounting],
            relativeTo: nil,
            bookmarkDataIsStale: &isStale
        ) {
            return Resolution(url: url, isStale: isStale)
        }

        return nil
    }

    /// Runs `body` with security-scoped access held, if this URL has any to hold.
    ///
    /// `startAccessingSecurityScopedResource()` returns `false` for a URL that is not
    /// security-scoped (a plain bookmark's, or anything already inside the container), and then
    /// there is nothing to stop — so the pairing stays balanced either way.
    static func withScopedAccess<T>(to url: URL, _ body: (URL) -> T) -> T {
        let started = url.startAccessingSecurityScopedResource()
        defer {
            if started {
                url.stopAccessingSecurityScopedResource()
            }
        }
        return body(url)
    }

    // MARK: - Persistence

    /// Now, at the resolution the file records — so the registry in memory and the registry on disk
    /// are *identical*, not merely close.
    ///
    /// Stamping through the same formatter the encoder uses makes the round trip exact by
    /// construction: what `string(from:)` produces, `date(from:)` maps back to the value that will
    /// be decoded, and that value formats to the same string again. Without this a freshly stamped
    /// record and its reloaded self would differ in the microseconds no format carries — an equality
    /// that fails only sometimes, which is the worst kind.
    private static func stamp(_ date: Date = Date()) -> Date {
        let formatter = boardRegistryTimestampFormatter()
        return formatter.date(from: formatter.string(from: date)) ?? date
    }

    /// Writes the whole file, atomically, and never throws.
    ///
    /// Whole-file because it is a handful of small records and a partial writer would be a
    /// consistency problem in exchange for nothing. Atomic because a crash mid-write must leave the
    /// previous file, not half of this one — the same temp-then-rename discipline `BoardWriter` uses
    /// on the user's own files. A failure is logged and swallowed: the caller is a window close or a
    /// settings toggle, and neither has anything useful to do about a full disk.
    private func save() {
        let encoder = JSONEncoder()
        // `.sortedKeys` for a byte-stable file (the same records always produce the same bytes) and
        // `.prettyPrinted` because the one time anybody reads this file by hand is when something
        // has gone wrong.
        encoder.outputFormatting = [.sortedKeys, .prettyPrinted]
        encoder.dateEncodingStrategy = .custom { date, encoder in
            var container = encoder.singleValueContainer()
            try container.encode(boardRegistryTimestampFormatter().string(from: date))
        }

        // Sorted by id, not by recency: the array's order carries no meaning (`recents()` sorts),
        // so pinning it keeps the file from churning every time a board is opened.
        let ordered = records.sorted { $0.id.uuidString < $1.id.uuidString }

        do {
            try FileManager.default.createDirectory(
                at: storageURL.deletingLastPathComponent(),
                withIntermediateDirectories: true
            )
            try encoder.encode(ordered).write(to: storageURL, options: .atomic)
            // Recorded *after* the write, so our own bytes never read as the sibling's on the next
            // `syncFromDiskIfChanged` — which is what keeps a single-edition launch behaving exactly as
            // it did before that mechanism existed.
            fileStamp = Self.fileStamp(of: storageURL)
        } catch {
            Self.logger.error("could not save the board registry: \(error.localizedDescription, privacy: .public)")
        }
    }

    private func loadFromDisk() -> [BoardRecord] {
        guard FileManager.default.fileExists(atPath: storageURL.path) else { return [] }

        guard let data = try? Data(contentsOf: storageURL) else {
            quarantine("unreadable")
            return []
        }

        let decoder = JSONDecoder()
        decoder.dateDecodingStrategy = .custom { decoder in
            let container = try decoder.singleValueContainer()
            let text = try container.decode(String.self)
            guard let date = boardRegistryTimestampFormatter().date(from: text) else {
                throw DecodingError.dataCorruptedError(
                    in: container,
                    debugDescription: "not an ISO 8601 timestamp: \(text)"
                )
            }
            return date
        }

        do {
            return try decoder.decode([BoardRecord].self, from: data)
        } catch {
            quarantine(error.localizedDescription)
            return []
        }
    }

    /// Renames the unreadable file aside so the next save starts clean.
    ///
    /// Renamed, never deleted: this file may be the only record of which boards a user had, and a
    /// bug that corrupts it should leave evidence to diagnose rather than a hole. The timestamp
    /// keeps repeat corruptions from overwriting each other; the id suffix covers two in the same
    /// millisecond.
    private func quarantine(_ reason: String) {
        Self.logger.error("board registry is unreadable (\(reason, privacy: .public)); quarantining it")

        // The timestamp's colons are legal in a path but read as `/` in Finder, so they go.
        let stamp = boardRegistryTimestampFormatter()
            .string(from: Date())
            .replacingOccurrences(of: ":", with: "-")
        let folder = storageURL.deletingLastPathComponent()
        let base = storageURL.deletingPathExtension().lastPathComponent
        let fileExtension = storageURL.pathExtension

        func destination(_ name: String) -> URL {
            let file = folder.appendingPathComponent(name, isDirectory: false)
            return fileExtension.isEmpty ? file : file.appendingPathExtension(fileExtension)
        }

        var target = destination("\(base)-corrupt-\(stamp)")
        if FileManager.default.fileExists(atPath: target.path) {
            target = destination("\(base)-corrupt-\(stamp)-\(UUID().uuidString.prefix(8))")
        }

        do {
            try FileManager.default.moveItem(at: storageURL, to: target)
        } catch {
            // Nothing further to do: the next `save()` overwrites the file atomically anyway, so a
            // failed quarantine costs the evidence, not the registry.
            Self.logger.error("could not quarantine it: \(error.localizedDescription, privacy: .public)")
        }
    }
}