lanework/Kanban/App/AppModel.swift

import AppKit
import Observation
import SwiftUI
import os

// MARK: - Scene ids

/// The scene identifiers, in one place because they are matched by string in three unrelated
/// spots — the scene declaration, `openWindow(id:)`, and `dismissWindow(id:)` — and a typo in any
/// one of them fails silently at runtime.
public enum WindowID {
    public static let welcome = "welcome"
    public static let restoreBootstrap = "restore-bootstrap"
    /// The template chooser (09-templates.md; File ▸ New Board… ⌥⌘N). Its own window rather than a
    /// sheet on welcome because ⌥⌘N is available *everywhere* (11-command-nexus.md) — including from
    /// a board window, and including when welcome is not open at all, which a sheet would have to
    /// conjure a host for.
    public static let templateChooser = "template-chooser"
    public static let board = "board"
    public static let card = "card"
}

// MARK: - App-wide preferences

/// The `UserDefaults` half of "App-wide state has the same home" (02-architecture.md § Per-board app
/// state): the app-scoped values that are scalars, kept out of the board registry because no board
/// owns them.
///
/// The keys are declared here rather than spelled at each `@AppStorage`, for the same reason
/// `WindowID` exists.
public enum AppPreferences {

    /// "Restore open boards at launch" (Settings, ⌘, — 11-command-nexus.md). **Default on.**
    public static let restoreOpenBoardsAtLaunchKey = "restoreOpenBoardsAtLaunch"

    /// Read outside a view, where `@AppStorage` is not available — the launch flow needs it before
    /// any scene exists. `object(forKey:)` rather than `bool(forKey:)` because the latter cannot
    /// tell "off" from "never set", and this preference defaults to *on*.
    public static var restoreOpenBoardsAtLaunch: Bool {
        UserDefaults.standard.object(forKey: restoreOpenBoardsAtLaunchKey) as? Bool ?? true
    }

    /// The last-used card-window size (05-card-window.md; 02 files it as app-wide, not per-board —
    /// "the last-used card-window size" is named there explicitly). Stored as a string because
    /// `NSSize` is not a property-list type and two more keys would be worse.
    public static let lastCardWindowSizeKey = "lastCardWindowSize"

    public static var lastCardWindowSize: CGSize? {
        guard let text = UserDefaults.standard.string(forKey: lastCardWindowSizeKey) else { return nil }
        let size = NSSizeFromString(text)
        guard size.width > 0, size.height > 0 else { return nil }
        return size
    }

    public static func setLastCardWindowSize(_ size: CGSize) {
        UserDefaults.standard.set(NSStringFromSize(size), forKey: lastCardWindowSizeKey)
    }

    /// The quick-style row's recently-used backgrounds — an array of palette names / hex strings,
    /// most-recent-first (03-board-ui.md § Styling ▸ Controls: "Recents are app-wide and persist
    /// app-side (user preference, never board data)"; 11-command-nexus.md files it under the
    /// preferences that "need no UI"). Read and written by `StyleRecents`, which owns the list rule;
    /// the key is declared here with its neighbours for `WindowID`'s reason.
    public static let quickStyleBackgroundsKey = "quickStyleBackgrounds"
}

// MARK: - Launch failures

/// A board that could not be restored or opened, as the welcome window renders it.
///
/// **A struct rather than the obvious tuple** only because SwiftUI needs identity to list these and
/// two failures can share a path (a board that failed, was retried, and failed again).
///
/// The join onto a recents row is `WelcomeRow.derive(recents:failures:)` — 02 § Launch and window
/// lifecycle wants the failure *on the board's row*, carrying fail-fast's specifics or the
/// unavailable state, and a failure naming no row (a first open of a folder that was never a board)
/// falls back to a list of its own. `path` is what the join matches on, which is why it is stored
/// rather than derived from the message.
public struct LaunchFailure: Identifiable, Sendable, Equatable {
    public let id = UUID()
    public let path: String
    public let message: String

    public init(path: String, message: String) {
        self.path = path
        self.message = message
    }

    /// What the row shows for a name: the folder, not the whole path. The path is the subtitle.
    public var displayName: String {
        URL(fileURLWithPath: path).deletingPathExtension().lastPathComponent
    }
}

// MARK: - Security-scoped access

/// One board's security-scoped access, held for the **whole session**.
///
/// `BoardRegistry.withScopedAccess(to:_:)` is the scoped-per-call form and is right for what it does
/// — resolving identities during a recents listing, where holding a scope open would be a leak. It is
/// exactly wrong for an open board: the store, the watcher, and every Writer call need access for
/// minutes or hours, and re-entering the scope per call would be both slower and racy against a
/// watcher thread that is already inside the folder.
///
/// So the pairing is explicit and its balance is the session's job: started when the board's window
/// opens, stopped in the close flush's teardown step. A class rather than a struct so the balance
/// cannot be duplicated by a copy.
///
/// **The URL matters, not the path.** A security-scoped URL is a token, not a string: a `URL`
/// rebuilt from `ref.path` grants nothing, which is why `AppModel.openBoard(at:)` stashes the
/// resolved URL for the host that is about to appear instead of letting it reconstruct one.
public final class ScopedAccess {

    public let url: URL
    private var started: Bool

    public init(_ url: URL) {
        self.url = url
        // `false` for a URL that is not security-scoped — a plain bookmark's, one the open panel
        // already blessed for the app's lifetime, anything inside the container. There is then
        // nothing to stop, and the pairing stays balanced either way.
        started = url.startAccessingSecurityScopedResource()
    }

    public func stop() {
        guard started else { return }
        started = false
        url.stopAccessingSecurityScopedResource()
    }
}

// MARK: - AppModel

/// The app's one piece of cross-window state: which boards are open, which card windows belong to
/// which board, and the two window actions AppKit-side code needs but cannot reach.
///
/// ### What lives here, and why it is not a singleton
///
/// The two registries (02-architecture.md § Layering ▸ Components and § Per-board app state) are
/// owned here because they are app-scoped and because "the app holds one instance, so a test can
/// hold its own without the two colliding" — `BoardStoreRegistry`'s own note. Everything else here is
/// window bookkeeping that has no other home: a `BoardStore` knows nothing about windows by design,
/// and a SwiftUI scene is a value that cannot hold state across a window's life.
///
/// ### Sessions are the join
///
/// A `BoardSession` is what makes the two halves of the app meet: the store the windows share, the
/// registry record they stamp, the card windows the close flush has to close first, and the
/// security-scoped access the whole thing runs inside. Its lifetime is exactly the board window's —
/// created when the host's load succeeds, removed by the close flush's last step. A card window with
/// no session is a card window with no board, which 02's ownership rule says cannot exist; the card
/// host reads that as "dismiss".
@MainActor
@Observable
public final class AppModel {

    // MARK: Registries

    public let storeRegistry = BoardStoreRegistry()
    public let boardRegistry: BoardRegistry

    /// The quick-style row's app-wide recents (03-board-ui.md § Styling ▸ Controls). Owned here for
    /// the registries' reason — app-scoped, and a test holds its own rather than colliding with the
    /// app's — and reached by the context menus through the environment, since a `BoardStore` is
    /// board-scoped and this list deliberately is not.
    public let styleRecents = StyleRecents()

    /// The app's one drag session (DRAG-REORDER.md; 04-interactions.md ▸ Drag and drop).
    ///
    /// App-wide for the reason cross-board drags exist at all: **a drag crosses windows**, so the
    /// source board hides the dragged items while any other open board's drop delegates propose a
    /// landing spot for them. It lives here rather than as a global for `styleRecents`' reason — a
    /// test holds its own rather than colliding with the app's — and every board window reaches it
    /// through the environment.
    /// Internal rather than `public`, unlike its neighbours: the drag is entirely a UI-layer
    /// concern, and nothing outside this module has any business reaching into a gesture in flight.
    let dragSession = DragSession()

    /// The app's one clipboard (04-interactions.md ▸ Clipboard).
    ///
    /// App-wide for the drag session's reason turned up a level: a cut/copy **outlives the board it
    /// came from** — the pasteboard and the staged snapshot survive the source window closing, and
    /// survive the app quitting — so nothing board-scoped could own it. It lives here rather than as
    /// a singleton for `styleRecents`' reason (a test holds its own rather than colliding with the
    /// app's, which for this one also means staying off the machine's real pasteboard), and every
    /// board window reaches it through the environment.
    ///
    /// Building it here is also the **launch sweep** (04: "a sweep at launch"): the store's `init`
    /// reads the pasteboard once and collects every staged tree it no longer names.
    public let clipboard = ClipboardStore()

    // MARK: The provider seam

    /// **The composition root for `HistoryProviding`** (12-editions.md ▸ The provider seam): what a
    /// board session's undo stack is built by, called once per board as its session begins.
    ///
    /// Base binds the native stack — an `NSUndoManager` over inverse `WriteOperation`s
    /// (13-native-undo.md) — and that is the default here because it is the *shared* code's
    /// implementation: both targets compile it, and Pro runs it too until pro-m1 replaces this
    /// closure with the git provider (06-history-undo.md). Nothing in this file is
    /// edition-conditional; the edition difference is which closure the root installs.
    ///
    /// It takes the store because that is what a provider is a history *of*: the git provider needs
    /// the board root it is a repository at, and the native one is about to need the same store's
    /// snapshots to compute inverses from. A property rather than an initializer argument so a test
    /// can bind a fake without a second `AppModel` initializer, `@ObservationIgnored` because
    /// nothing renders from it.
    @ObservationIgnored
    public var makeHistoryProvider: (BoardStore) -> any HistoryProviding = { _ in NativeHistoryProvider() }

    // MARK: Sessions

    /// One open board window and everything hanging off it.
    public struct BoardSession {

        /// The shared store — the same object every one of this board's windows renders.
        public let store: BoardStore

        /// Which registry record this board is, so the close flush can stamp counts and clear the
        /// open-now flag without matching by identity a second time.
        public let recordID: UUID

        /// This board's undo/redo substrate — **one stack per board session, never per window**
        /// (13-native-undo.md ▸ Rules). It lives here for the store's reason exactly: the session is
        /// what every window over this board shares, and "undo is board-local".
        ///
        /// Which implementation it is, is the edition's answer and nobody else's
        /// (12-editions.md ▸ The provider seam) — see `AppModel.makeHistoryProvider`.
        public let history: any HistoryProviding

        /// The same stack, wearing the face AppKit needs (`BoardUndoManager`): what this board's
        /// windows hand back from `windowWillReturnUndoManager`, so the Edit menu's Undo/Redo rows
        /// and the toolbar's pair resolve to *this* board through the ordinary responder chain.
        ///
        /// Built once with the session rather than per window, because a second adapter would be a
        /// second answer to "what is this board's undo" — and card windows share this one.
        let undoManager: BoardUndoManager

        /// This board's open card windows. The close flush's step 1 reads it; the card hosts
        /// maintain it. Empty is the common case.
        public var cardRefs: Set<CardWindowRef> = []

        /// The scope the board is being read and written inside, released at teardown. `nil` when
        /// the board was opened from a URL that needed none.
        var access: ScopedAccess?
    }

    /// Keyed by board window, because that is the thing whose lifetime a session shares.
    ///
    /// Observed: a card window watches for its board's session disappearing and dismisses itself when
    /// it does — the safety net behind "card windows never outlive the board window".
    public private(set) var sessions: [BoardWindowRef: BoardSession] = [:]

    /// The end-session hooks, keyed the same way the card windows are.
    ///
    /// Beside `BoardSession.cardRefs` rather than inside it: the set is *membership* (what the close
    /// flush drains and what the safety net checks), this is the *seam table* (what it calls). They
    /// are only ever written together, by the two register/unregister methods below, which is what
    /// keeps them from becoming two answers to one question.
    @ObservationIgnored
    private var cardSessions: [CardWindowRef: any CardSessionFlushing] = [:]

    /// Boards whose close flush is already running — the re-entrancy guard.
    ///
    /// Needed because a board window can be told to close twice in quick succession: the
    /// `windowShouldClose` interception starts the flush, and the host's own disappear runs a second
    /// attempt as its safety net. The second must not re-enter a sequence that is mid-await.
    @ObservationIgnored
    private var closingBoards: Set<BoardWindowRef> = []

    // MARK: Window actions

    /// SwiftUI's window-opening action, captured from whatever scene view is alive.
    ///
    /// It exists because the two things that most need to open a window are not views:
    /// `AppDelegate.applicationShouldHandleReopen` (a Dock click with no windows must show welcome)
    /// and the close-flush coordinator (which dismisses card windows). Neither can read
    /// `@Environment`. The action stays valid after the view that supplied it is gone — it is a value
    /// addressed to the app, not to a window — which is precisely the windowless case it is for.
    ///
    /// `@ObservationIgnored` on both: nothing renders from them, and an assignment on every scene's
    /// appear would otherwise invalidate every observer for no reason.
    @ObservationIgnored
    public var windowOpener: OpenWindowAction?

    @ObservationIgnored
    public var windowDismisser: DismissWindowAction?

    /// URLs handed to `openBoard(at:)` before `windowOpener` existed to open them — a cold launch's
    /// Finder-open (`AppDelegate.application(_:open:)`) can arrive ahead of the first scene's
    /// `onAppear`. Held in order, replayed the moment `captureWindowActions` gives the app somewhere
    /// to open them, then discarded — the buffer is a doorway, not a second registry of intent.
    @ObservationIgnored
    private var pendingOpenURLs: [URL] = []

    /// What `CaptureOpenWindow` calls. A method rather than two assignments so the launch flow, which
    /// needs the actions before any `onAppear` has run, has one thing to call.
    func captureWindowActions(open: OpenWindowAction, dismiss: DismissWindowAction) {
        windowOpener = open
        windowDismisser = dismiss

        guard !pendingOpenURLs.isEmpty else { return }
        let urls = pendingOpenURLs
        pendingOpenURLs.removeAll()
        for url in urls {
            openBoard(at: url)
        }
    }

    // MARK: Recents

    /// The recents list, cached: what the welcome window renders and what File ▸ Open Recent lists
    /// (02-architecture.md § Per-board app state — "The recents list *is* this registry sorted by
    /// last-opened").
    ///
    /// **Cached rather than read through on demand, and both halves of that are deliberate.**
    /// `BoardRegistry` is not `@Observable`, so a view reading it directly would never learn that a
    /// row was forgotten; and `recents()` resolves every record's bookmark, which is filesystem work
    /// no SwiftUI body should be doing on every evaluation — the File menu's command graph is
    /// rebuilt far more often than this list changes. So the list lives here as observable state and
    /// every path that can change the registry refreshes it explicitly (`refreshRecents()`).
    ///
    /// The honest residual: a registry mutated behind this object's back would show stale until the
    /// next refresh. There is no such path today — every writer goes through this type or through a
    /// session it owns — and welcome refreshes on appearance as the cheap belt-and-braces.
    public private(set) var recents: [RecentBoard] = []

    /// Re-reads the registry into `recents`. Called wherever the registry changes: a board opening,
    /// a board closing (the counts are stamped there), Forget, Clear Menu, and welcome appearing.
    public func refreshRecents() {
        recents = boardRegistry.recents()
    }

    /// The welcome row's Forget (11-command-nexus.md ▸ Welcome recent) — the record, plus any launch
    /// failure that row was carrying, plus the refresh, in one call so no caller can do one without
    /// the others.
    ///
    /// **Forgetting the board forgets the failure too.** The row *is* the failure's surface (02
    /// § Launch and window lifecycle); dropping the row while keeping the failure would relocate its
    /// message into the unmatched-failures list, which reads as the app declining to forget.
    public func forget(boardID: UUID) {
        clearLaunchFailures(naming: knownPaths(ofBoard: boardID))
        boardRegistry.forget(id: boardID)
        refreshRecents()
    }

    /// File ▸ Open Recent ▸ Clear Menu (11-command-nexus.md).
    ///
    /// **Finder clears the *menu*; here the registry is the menu**, so clearing removes every record
    /// — there is no second list to clear, and a "menu" that still knew about the boards it had
    /// stopped listing would be a distinction with no surface. What that costs is per-board settings
    /// (window frames, push-on-commit) for boards the user reopens later, which is exactly what
    /// Forget costs one row at a time and what 02's "its settings are conveniences" already accepts.
    ///
    /// It is Forget applied wholesale, so it clears failures the same way — the ones naming records,
    /// leaving a failure that named no row (and therefore no menu entry) standing in its own list.
    ///
    /// A board that is open right now keeps working: its session holds a record id that no longer
    /// resolves, and `BoardRegistry.update` treats an unknown id as a no-op for precisely this case.
    public func clearRecents() {
        clearLaunchFailures(naming: Set(recents.flatMap { recent in
            [recent.record.lastKnownPath, recent.url?.path].compactMap { $0 }
        }))
        boardRegistry.forgetAll()
        refreshRecents()
    }

    /// Every path a given record is known by — the one it was last seen at and, when its bookmark
    /// still resolves, where it lives now. The two can differ (a bookmark follows a move), and a
    /// failure recorded before the move names the older one.
    private func knownPaths(ofBoard id: UUID) -> Set<String> {
        var paths: Set<String> = []
        if let record = boardRegistry.record(id: id) {
            paths.insert(record.lastKnownPath)
        }
        if let url = recents.first(where: { $0.record.id == id })?.url {
            paths.insert(url.path)
        }
        return paths
    }

    // MARK: Launch failures

    /// Boards that failed to restore or open, newest last. Rendered on their own recents rows where
    /// one exists, and in a fallback list where none does — `WelcomeRow.derive(recents:failures:)`.
    public private(set) var launchFailures: [LaunchFailure] = []

    // MARK: Card-window placement

    /// Where the next card window cascades from (05-card-window.md, "New windows open at the
    /// last-used card-window size, cascaded").
    ///
    /// `NSWindow.cascadeTopLeft(from:)` is the whole mechanism: passing `.zero` places the window at
    /// its natural position and returns the point for the next one, so this is a running cursor
    /// rather than a computed grid. App-wide, not per-board: two boards' card windows cascade past
    /// each other rather than landing on top of one another.
    @ObservationIgnored
    var cardCascadePoint: NSPoint = .zero

    // MARK: Pending opens

    /// The security-scoped URL a board window is about to be built from, stashed between
    /// `openBoard(at:)` and the host's first appearance.
    ///
    /// The handoff exists because a window value has to be `Codable` and a scoped URL is not a
    /// string: by the time `BoardWindowHost` receives its `BoardWindowRef` the access token is gone
    /// unless something carried it across. The host claims it on appear; an unclaimed entry (a window
    /// that never opened) leaks one scope until quit, which is the cheapest failure available here.
    @ObservationIgnored
    private var pendingAccess: [BoardWindowRef: ScopedAccess] = [:]

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

    /// The app builds one of these with the real registry file; a test passes its own path for the
    /// same reason `BoardRegistry` takes one at all — "injecting it is how a test stays out of the
    /// real Application Support directory".
    public init(registryStorageURL: URL = BoardRegistry.defaultStorageURL) {
        boardRegistry = BoardRegistry(storageURL: registryStorageURL)
        // Read once here rather than lazily, so File ▸ Open Recent is populated from the app's first
        // menu pass — a launch that restores boards never shows welcome, and a submenu that filled
        // in only after the first close would look broken. It costs one bookmark-resolution sweep at
        // launch, next to the one `restorables()` already runs.
        refreshRecents()
    }

    // MARK: - Launch restoration

    /// The launch-restoration gate, as a pure function (02-architecture.md § Launch and window
    /// lifecycle: "the preference gates only whether the flagged set is consulted; the flags are
    /// maintained regardless").
    ///
    /// `KanbanApp.init()` is where this actually runs — read once, before any scene exists, into a
    /// `let` rather than a computed property, because `restorables()` costs a bookmark resolution per
    /// known board and nothing should pay that on every scene-graph evaluation. An `App`'s `init` is
    /// not itself reachable from a test, so the decision is pulled out to here: two `Bool`s in, one
    /// out, provable without a real `UserDefaults` domain or a live registry.
    public static func shouldRestoreAtLaunch(preference: Bool, hasRestorables: Bool) -> Bool {
        preference && hasRestorables
    }

    // MARK: - Opening

    public var hasOpenBoards: Bool { !sessions.isEmpty }

    /// Opens a board window for `url`, or focuses the one this board already has.
    ///
    /// **The already-open check is by file identity, not by path** — `liveStore(for:)` resolves it —
    /// so a board reached through a resolved bookmark and the same board reached through the open
    /// panel land on one window even when the two URLs are spelled differently. Only when nothing is
    /// open for it does a ref get minted, and `openWindow(value:)` with an equal ref focuses rather
    /// than duplicates, which is the second half of "one board window per root".
    ///
    /// Security-scoped access starts here, *before* the window exists, because the host's very first
    /// act is a tree walk: a scope started after the load would be too late.
    ///
    /// **Called before any scene has appeared, and that's fine.** A cold launch's Finder-open can
    /// reach here before `windowOpener` is captured; the URL joins `pendingOpenURLs` and this same
    /// method runs again for it once `captureWindowActions` has something to open it with.
    public func openBoard(at url: URL) {
        guard let windowOpener else {
            pendingOpenURLs.append(url)
            return
        }

        if storeRegistry.liveStore(for: url) != nil, let existing = boardRef(forBoardAt: url) {
            windowOpener(id: WindowID.board, value: existing)
            return
        }

        let ref = BoardWindowRef(url: url)
        // Replacing a stash for the same ref would strand the old scope; there is no such case today
        // (an unopened window's ref is not reachable), but stopping the loser is free.
        pendingAccess.removeValue(forKey: ref)?.stop()
        pendingAccess[ref] = ScopedAccess(url)
        windowOpener(id: WindowID.board, value: ref)
    }

    /// Shows — or focuses — the welcome window. Its own scene id, so this works with no windows at
    /// all, which is the Dock-reactivation case (02: "Reactivation (Dock click) with no windows shows
    /// welcome").
    public func showWelcome() {
        windowOpener?(id: WindowID.welcome)
    }

    /// File ▸ New Board… (⌥⌘N) — shows, or focuses, the template chooser (09-templates.md).
    ///
    /// The command opens a *chooser*, never a board: the location is the save panel's question and
    /// the panel is the chooser's, so this method's whole job is the window.
    public func showTemplateChooser() {
        windowOpener?(id: WindowID.templateChooser)
    }

    /// The standard open panel behind File ▸ Open… ⌘O (11-command-nexus.md).
    ///
    /// **Validation is the open attempt itself** — there is no pre-flight check that a folder is a
    /// board. Fail-fast owns that verdict (01-storage-format.md § Malformed input) and it is the same
    /// verdict a restored board gets, so a folder that is not a board produces one error in one
    /// vocabulary rather than two near-identical rejections in two.
    ///
    /// `treatsFilePackagesAsDirectories` is what lets a `.kanban` package be *chosen* while
    /// `canChooseFiles` stays off: a package is a file to the panel otherwise, and boards are both
    /// packages and plain folders (01 § Board naming). The cost is that double-clicking a package
    /// navigates into it, which the welcome window's own open affordances will make moot.
    public func presentOpenPanel() {
        let panel = NSOpenPanel()
        panel.canChooseDirectories = true
        panel.canChooseFiles = false
        panel.treatsFilePackagesAsDirectories = true
        panel.allowsMultipleSelection = false
        panel.prompt = "Open"
        panel.message = "Choose a board folder."

        guard panel.runModal() == .OK, let url = panel.url else { return }
        openBoard(at: url)
    }

    /// The ref of the window already showing the board at `url`, if any — matched through the store,
    /// which is identity-keyed, rather than through the path.
    private func boardRef(forBoardAt url: URL) -> BoardWindowRef? {
        guard let store = storeRegistry.liveStore(for: url) else { return nil }
        return sessions.first { $0.value.store === store }?.key
    }

    // MARK: - Sessions

    public func session(for ref: BoardWindowRef) -> BoardSession? {
        sessions[ref]
    }

    /// Claims the scoped URL `openBoard(at:)` stashed for this window, or `nil` if it opened by some
    /// other route. Claiming removes it: the session owns the balance from here.
    func claimPendingAccess(for ref: BoardWindowRef) -> ScopedAccess? {
        pendingAccess.removeValue(forKey: ref)
    }

    /// Starts a board's session — the board window's host calls this once its load has succeeded.
    ///
    /// Two bookkeeping consequences of "this board is now open" ride along. The recents list is
    /// re-read, because `recordOpen` just moved this board to the top of it. And any launch failure
    /// naming this board is dropped: the board demonstrably opens, so a row still captioned with the
    /// old error would be reporting a condition that has stopped being true. That is not the silent
    /// drop 02 forbids — it forbids a failure that was never surfaced disappearing, not one the user
    /// has since fixed.
    func beginSession(ref: BoardWindowRef, store: BoardStore, recordID: UUID, access: ScopedAccess?) {
        // The board's stack is born here, with the session that owns it, and dies in `tearDown`
        // below — the whole of 13-native-undo.md's session-only persistence: "the stack lives with
        // the board session and dies at close/quit ... standard macOS behavior".
        let history = makeHistoryProvider(store)
        // **The binding 13-native-undo.md ▸ Rules' "registration at the Writer boundary" needs**: the
        // store is that boundary — every app-mediated mutation goes out through one of its write
        // methods — so it is the store that computes each inverse and registers it. What it cannot
        // know is *which* stack, because a stack belongs to a session and a store knows nothing about
        // windows; this line is where the session tells it. Weak on the store's side, so the loop
        // this closes (provider → step closures → store) is not a retain cycle.
        store.history = history
        sessions[ref] = BoardSession(
            store: store,
            recordID: recordID,
            history: history,
            // The lock's enablement half (13-native-undo.md ▸ Rules): Undo and Redo disable with the
            // other mutating commands while the board refuses writes, and the stack survives to
            // resume when it clears. Weak, so the adapter is never the reason a closed board's store
            // stays alive; a store that has gone answers "writable", which is moot — its stack went
            // with it.
            undoManager: BoardUndoManager(history: history, isReadOnly: { [weak store] in
                store?.isReadOnly ?? false
            }),
            cardRefs: [],
            access: access
        )
        clearLaunchFailures(naming: [ref.path, store.rootURL.path])
        refreshRecents()
    }

    /// Registers a card window with its board's session, so the close flush can find it.
    ///
    /// A card window whose board has no session is a card window with no board — the ownership rule
    /// says that cannot exist, and the host's own check dismisses it before reaching this. Recording
    /// the seam anyway would leave an entry nothing ever drains.
    func registerCardWindow(_ ref: CardWindowRef, session: any CardSessionFlushing) {
        guard sessions[ref.board] != nil else {
            Self.logger.debug("card window registered against a board with no session — ignored")
            return
        }
        sessions[ref.board]?.cardRefs.insert(ref)
        cardSessions[ref] = session
    }

    func unregisterCardWindow(_ ref: CardWindowRef) {
        sessions[ref.board]?.cardRefs.remove(ref)
        cardSessions[ref] = nil
    }

    // MARK: - Launch failures

    /// Records a board that could not be opened. Deliberately additive and never cleared on success:
    /// welcome is showing *because* something failed, and a list that emptied itself as other boards
    /// arrived would be the silent drop 02 rules out.
    public func recordLaunchFailure(path: String, message: String) {
        launchFailures.append(LaunchFailure(path: path, message: message))
    }

    /// Forgets the failures — the welcome window's dismissal of a list the user has read.
    public func clearLaunchFailures() {
        launchFailures.removeAll()
    }

    /// Forgets exactly the named failures — what the unmatched-failures list's Clear dismisses, so
    /// that pressing it never also erases a message still standing on a recents row the user has
    /// not looked at.
    public func clearLaunchFailures(ids: Set<UUID>) {
        launchFailures.removeAll { ids.contains($0.id) }
    }

    /// Drops every failure naming one of `paths` — the resolution path, used when a board opens
    /// successfully and when its record is forgotten. Paths are compared the way the welcome row's
    /// join compares them, so "this row's failure" means the same thing in both places.
    private func clearLaunchFailures(naming paths: Set<String>) {
        guard !paths.isEmpty else { return }
        let keys = Set(paths.map(WelcomeRow.pathKey))
        launchFailures.removeAll { keys.contains(WelcomeRow.pathKey($0.path)) }
    }

    // MARK: - Counts

    /// The lane and card counts stamped into the registry at close — **live items only** (02
    /// § Per-board app state, settled).
    ///
    /// > tombstoned lanes and cards — and cards hidden beneath a tombstoned lane — don't count; the
    /// > row advertises the board's working size, and the trash is an errand, not inventory.
    ///
    /// The nesting is the ancestor walk: a tombstoned lane is skipped whole, so its cards are never
    /// reached whatever their own flags say. `Lane.isDeleted`/`Card.isDeleted` are presence-of-key,
    /// not validity, so a malformed `deleted:` counts as deleted here exactly as it does everywhere
    /// else.
    ///
    /// Static and pure: it is a fact about a snapshot, and the close flush is the wrong place to
    /// discover a counting bug.
    public static func liveCounts(of snapshot: BoardModel) -> (lanes: Int, cards: Int) {
        var lanes = 0
        var cards = 0
        for lane in snapshot.lanes where !lane.isDeleted {
            lanes += 1
            for card in lane.cards where !card.isDeleted {
                cards += 1
            }
        }
        return (lanes, cards)
    }

    /// The folder name, extension stripped (01-storage-format.md § Board naming) — `displayName`'s
    /// own fallback, and (02-architecture.md § Per-board app state) the registry record's
    /// *provisional* display name for a board recorded before its load has run: "fail-fast means the
    /// frontmatter can't be trusted, and the folder name is the Finder document name the user just
    /// picked". A first successful load replaces it with the cached title through the ordinary
    /// `displayName(of:)` path — there is no separate provisional vocabulary, just this one fallback
    /// used a moment earlier than usual.
    public static func folderDisplayName(of url: URL) -> String {
        url.deletingPathExtension().lastPathComponent
    }

    /// A board's display name: its `title`, falling back to the folder name sans extension
    /// (01-storage-format.md § Board naming).
    ///
    /// Read from `store.rootURL` rather than `snapshot.rootURL` so the fallback follows a rename the
    /// moment it is absorbed, instead of lagging by one reload (see `BoardStore.rootURL`).
    public static func displayName(of store: BoardStore) -> String {
        if let title = store.snapshot.title.value, !title.isEmpty {
            return title
        }
        return folderDisplayName(of: store.rootURL)
    }

    // MARK: - Closing

    /// Runs the close flush for one board and tears its session down.
    ///
    /// Idempotent by two guards: a board with no session has already closed, and a board already
    /// mid-flush is not started again. Both matter — the window's close interception and the host's
    /// disappear both call this, by design, because neither one alone fires on every path a window
    /// can leave by.
    public func closeBoard(ref: BoardWindowRef, cause: BoardCloseCause) async {
        guard sessions[ref] != nil, !closingBoards.contains(ref) else { return }
        closingBoards.insert(ref)
        defer { closingBoards.remove(ref) }

        await coordinator(for: ref).run(cause: cause)
        // The flush stamped this board's counts and (on a user close) cleared its open-now flag, so
        // the cached list is now one close out of date — and welcome is often the very next thing on
        // screen.
        refreshRecents()
    }

    /// The close flush's **pending-work step, without the teardown** — what File ▸ Duplicate runs
    /// before it copies (03-board-ui.md § Welcome screen & templates: "The copy is preceded by the
    /// close flush ... so neither the tree nor the copied history misses pending work").
    ///
    /// **Not `closeBoard`**, and the design says so itself: 09-templates.md ▸ Save as Template states
    /// the rule together with its exception — "with the pull-style mechanical exception committing an
    /// open Edit session's on-disk saves as-is, **sessions staying open**". A duplicate leaves the
    /// original on screen (03: "the original stays open too"), so what it needs is pending work
    /// *landed on disk*, not a session ended: no card window is dismissed, no record is stamped
    /// closed, nothing is torn down, and the board the user is looking at never blinks.
    ///
    /// It goes through `CloseFlushCoordinator` rather than calling the store directly so that the
    /// order of the three flushes — store pipeline, then editor saves, then the pending auto-commit
    /// (02's own order) — keeps having exactly one definition.
    public func flushPendingWork(for ref: BoardWindowRef) async {
        guard sessions[ref] != nil else { return }
        await coordinator(for: ref).flushPendingWork()
    }

    /// Quit: the same sequence, once per open board, **sequentially**.
    ///
    /// Sequential rather than concurrent so each board's ordering is the one 02 fixes rather than
    /// three interleavings of it, and in a stable board order so a quit is reproducible. Nothing here
    /// clears an open-now flag — that is what `.quit` means, and it is what makes the next launch
    /// restore this set (§ Launch and window lifecycle).
    public func flushAllBoardsForQuit() async {
        for ref in sessions.keys.sorted(by: { $0.path < $1.path }) {
            await closeBoard(ref: ref, cause: .quit)
        }
    }

    /// Wires a session into `CloseFlushCoordinator`'s seams. The ordering lives over there; this is
    /// only which real object each step touches.
    private func coordinator(for ref: BoardWindowRef) -> CloseFlushCoordinator {
        CloseFlushCoordinator(
            openCardRefs: { [weak self] in
                // Sorted so a board with several card windows commits and closes them in a stable
                // order rather than a `Set`'s.
                (self?.sessions[ref]?.cardRefs).map { $0.sorted { $0.cardID < $1.cardID } } ?? []
            },
            endCardSession: { [weak self] cardRef in
                await self?.cardSessions[cardRef]?.endSession()
            },
            dismissCardWindow: { [weak self] cardRef in
                self?.windowDismisser?(value: cardRef)
            },
            storeFlush: { [weak self] in
                await self?.sessions[ref]?.store.awaitQuiescence()
            },
            // `editorFlush` stays nil, and now deliberately rather than for want of an editor: the
            // card windows' debounced body saves flush in **step 1**, inside each window's
            // `endSession()` (`CardWindowSession`), which is both earlier than this slot and where
            // 02-architecture.md puts them ("each open Edit session ends with its normal session
            // commit", then pending work). The slot stays for a board-level editor with no card
            // window of its own — the raw-source buffer is the candidate — so that the order
            // relative to `committerFlush` (m7) is already decided when one arrives.
            recordClose: { [weak self] in
                guard let self, let session = sessions[ref] else { return }
                let counts = Self.liveCounts(of: session.store.snapshot)
                boardRegistry.recordClose(
                    id: session.recordID,
                    displayName: Self.displayName(of: session.store),
                    laneCount: counts.lanes,
                    cardCount: counts.cards,
                    icon: session.store.snapshot.icon.value,
                    iconColor: session.store.snapshot.iconColor.value
                )
            },
            clearOpenNow: { [weak self] in
                guard let self, let session = sessions[ref] else { return }
                boardRegistry.clearOpenNow(id: session.recordID)
            },
            tearDown: { [weak self] in
                guard let self, let session = sessions.removeValue(forKey: ref) else { return }
                // Session-only persistence, the other half of `beginSession` (13-native-undo.md
                // ▸ Rules): "the stack ... dies at close/quit", so reopening the board starts empty.
                // Cleared rather than merely dropped because the steps hold closures over the store
                // this line is about to release, and a stack that outlived its board would be a
                // retain cycle wearing an undo stack's clothes.
                session.history.clear()
                storeRegistry.release(session.store)
                session.access?.stop()
            }
        )
    }
}