import Foundation
import os

/// Walks a board's folder tree and produces an immutable `BoardModel` snapshot — a pure
/// function of the tree (02-architecture.md § Layering ▸ Components). Enforces the fractal
/// layout's fail-fast and skip rules (01-storage-format.md § Fractal layout, Malformed input)
/// so a bad file either loudly rejects the whole load or is cleanly ignored — never a silent
/// partial result.
///
/// Level is position: root `index.md` → board, depth-1 folders → lanes, depth-2 folders →
/// cards. **Name shape gates level detection** (01-storage-format.md § Fractal layout ▸
/// Rules): only a folder whose name has a UUID's shape — hex, `8-4-4-4-12`, **any case and any
/// version** — is a lane/card *candidate* at those depths; see `isUUIDShaped` below for exactly
/// what's checked.
/// Anything else — even a directory holding a perfectly valid `index.md` — is a stray: skipped
/// with a `.nonUUIDFolderIgnored` warning, preserved verbatim on disk, and never descended
/// into. A hand-made `notes/` folder (or a broken `index.md` inside one) can never brick a
/// load; only a UUID-shaped candidate that is itself missing `index.md` still gets the older
/// `.missingIndex` warning, and only a UUID-shaped candidate's `index.md` can fail-fast.
///
/// Reserved child names (`attachments/`, `comments/`) only matter as children *of a card*
/// (01-storage-format.md § Fractal layout ▸ Rules), and cards are leaves *structurally*: the
/// walk stops at depth 2, so nothing below a card is ever a level candidate. Doubly so under the
/// shape rule — `attachments` and `comments` are non-UUID-shaped and would read as strays, not
/// levels, so they never need special-casing against the stray warning.
///
/// **Two reads inside a card folder**, both of them flat name listings and nothing more — neither
/// opens a file, descends, warns, or fails a load; each degrades to `[]`:
///
/// - `attachmentNames(in:)` — `attachments/`, feeding `Card.attachments`. The board window's face
///   needs it before a card window exists (the quiet paperclip indicator — 03-board-ui.md § Card
///   face), and the snapshot is where it reads from.
/// - `looseFileNames(in:)` — the card folder *itself*, feeding `LoadResult.looseCardFiles`. This is
///   the loose-file carve-out's **detection** half (01-storage-format.md § Fractal layout ▸ Rules,
///   settled 2026-07-28): a regular file sitting beside a card's `index.md` belongs in
///   `attachments/`, and the app relocates it. Detection stays read-only *here* — this loader is a
///   pure function of the tree and writes nothing, ever (the Repair precedent); the relocation is a
///   Writer-mediated app write the store schedules off the snapshot
///   (`BoardStore.relocateLooseCardFiles`).
///
/// Everything else about a card folder's contents remains outside this loader's business.
///
/// Symlinks: a lane/card candidate that is itself a symlink is treated as a stray and never
/// followed, whether it points to a file or a directory — this loader does not resolve
/// cross-volume or cyclic trees.
///
/// ## The trash is a container, not a level
///
/// `<root>/.trash/` is a **reserved, app-claimed board-root name** holding card folders directly
/// (01-storage-format.md § Deletion, resettled 2026-07-28) — "same shape as a lane's children, no
/// `index.md` of its own". The walk therefore treats it as a second card container beside the
/// lanes: `trashCards(in:)` parses its UUID-shaped children with exactly the card parse the lane
/// walk uses (same fail-fast on `schema`/`order`, same skip-and-warn rules), and the result lands
/// in `BoardModel.trash` rather than under any lane. Being reserved, it is **never a stray** and
/// never warns; absent, the trash is simply empty.
///
/// ## The migration window
///
/// The tombstone model is retired: no `deleted:` key is ever written again, and a key found on
/// load is *migration input* — a card relocates into `.trash/` with the key removed, a lane
/// returns live with the key removed, a board-level key stays meaningless (ignored + logged).
/// Detection is read-only here, the loose-file carve-out's posture exactly: this loader reports
/// what it found through `LoadResult.legacyTombstones` and the store schedules the Writer-mediated
/// fix.
///
/// **Until that write lands, such items still load through the retiring tombstone path** —
/// a `deleted:`-carrying lane or card stays filed under its lane with `isDeleted` set. That is a
/// deliberate intermediate, not an oversight: the migration's whole promise is "never destroy",
/// and the safe direction while the fix is still pending (it is deferred under any read-only lock,
/// and can be pending for a whole session) is for nothing to silently vanish from view before its
/// folder has actually moved. The window closes per board on the first successful migration write,
/// after which no `deleted:` key exists to read and the flag is permanently `false`.
public enum BoardLoader: Sendable {

    /// Schema version this app understands; anything higher fails fast
    /// (01-storage-format.md § Malformed input). Internal rather than `private`: read by
    /// `BoardLoadError.Reason.description` below, and it is also the version `BoardWriter`
    /// stamps into files it creates — one symbol, so the app can never write a file its own
    /// loader would reject as newer-than-supported.
    static let supportedSchema = 1

    /// Board-level key for `BoardModel.template` — not schema-owned in the engine's sense
    /// (`FrontmatterKeys.schemaOwned`), because its value is opaque and read raw here rather
    /// than through a typed `FrontmatterDocument` accessor.
    ///
    /// Internal rather than `private` for `indexFileName`'s reason: `TemplateEngine` writes this
    /// key on a Save as Template copy — "the one writer of keyed files is Save as Template"
    /// (09-templates.md ▸ Storage) — and the reader and that one writer must never disagree about
    /// how it is spelled.
    static let templateKey = "template"

    /// Internal rather than `private`: `BoardWriter` names the same file, and the loader and
    /// the writer must never disagree about which file a folder's content lives in.
    ///
    /// The name itself is `IntegrityRules`', with every other reserved name — one table
    /// (02-architecture.md ▸ Components).
    static let indexFileName = IntegrityRules.indexFileName

    /// The materialized trash container at board root (01-storage-format.md § Deletion, resettled
    /// 2026-07-28) — **app-claimed, never a stray**, joining `CLAUDE.md`, `CLAUDE.user.md` and the
    /// seeded `.gitignore` on the claimed list.
    ///
    /// Dot-prefixed, which is doing real work rather than being decoration: `directoryCandidates`
    /// skips hidden entries, so the container can never be mistaken for a lane candidate and can
    /// never earn a `.nonUUIDFolderIgnored` warning. The lane walk skips it by name as well
    /// (`reservedRootNames`) so the rule holds even where hidden-file semantics don't.
    ///
    /// Internal rather than `private`: `BoardWriter` moves folders into and out of this exact
    /// name, and a board can have only one trash. The name is `IntegrityRules`', with the rest of
    /// the claimed-name table.
    static let trashFolderName = IntegrityRules.trashFolderName

    /// Board-root names the app claims, and therefore the names the lane walk skips **without a
    /// stray warning** (01-storage-format.md § Fractal layout ▸ Rules: "Three board-root names are
    /// app-claimed, not strays", plus `.trash/` from § Deletion).
    ///
    /// Only `.trash` is a *folder* and so the only one the directory walk could ever reach; the
    /// files are listed because the claim is about names, and a future check that needs the set
    /// should find it complete rather than build a second one. Compared lowercased, like
    /// `reservedCardChildNames` and for its reason — the filesystem this runs on usually is.
    ///
    /// The table is `IntegrityRules.claimedRootNames`, which also carries what kind of node each
    /// name is allowed to be — the fact the squatter-displacement heal turns on (ruled 2026-07-29).
    static let reservedRootNames: Set<String> = IntegrityRules.claimedRootNameSet

    /// The card-level names the app claims, and therefore the three the loose-file carve-out
    /// never touches (01-storage-format.md § Fractal layout ▸ Rules: "Reserved card-level names
    /// … untouched"): the card's own `index.md` plus the two reserved children. `comments` is
    /// listed because the schema reserves the name, not because anything writes it yet —
    /// `attachments/` is still the one folder this app ever creates under a card.
    ///
    /// **Compared lowercased**, because the filesystem this runs on usually is: a file spelled
    /// `Index.md` *is* the card's index to `fileExists`, and a case-sensitive reservation check
    /// would hand the loose-file relocation a card's own content to move into `attachments/`.
    ///
    /// Internal rather than `private`: `BoardWriter.relocateLooseFiles` refuses the same three
    /// names on its own, so a caller passing a hand-made list cannot reach past this rule. The
    /// table itself is `IntegrityRules`', with every other reserved name.
    static let reservedCardChildNames: Set<String> = IntegrityRules.reservedCardChildNames

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

    // MARK: - Entry point

    public static func load(
        boardRoot: URL,
        historyRanker: IdentityHistoryRanker? = nil
    ) throws(BoardLoadError) -> LoadResult {
        try checkIsReadableDirectory(boardRoot)

        let boardIndexURL = boardRoot.appendingPathComponent(indexFileName)
        guard FileManager.default.fileExists(atPath: boardIndexURL.path) else {
            throw BoardLoadError(path: indexFileName, reason: .boardRootMissingIndex)
        }
        let boardDocument = try readDocument(at: boardIndexURL, path: indexFileName)
        let boardSchema = try validatedSchema(in: boardDocument, path: indexFileName)

        var warnings: [LoadWarning] = []
        func warn(_ warning: LoadWarning) {
            warnings.append(warning)
            logger.warning("\(warning.description, privacy: .public)")
        }

        // **The one typed defect stream** (02-architecture.md ▸ Components ▸ IntegrityRules): what
        // this walk found that is pending *work*, as distinct from `warnings`, which is the
        // stray-*tolerance* vocabulary — information, not work. The two ad-hoc repair channels this
        // replaced (loose files, legacy tombstones) are still readable under their own names as
        // views over it (`LoadResult.looseCardFiles`, `.legacyTombstones`).
        var defects: [IntegrityRules.Defect] = []

        // Detected before the walk, so a board whose `.trash` is squatted reports it even though
        // the trash read below finds nothing to parse. Read-only here, like every other detection:
        // the displacement is the store's, through the Writer (the Repair precedent).
        if let squatter = IntegrityRules.squattedClaimedName(atBoardRoot: boardRoot) {
            defects.append(.claimedNameSquatted(squatter))
            logger.warning(
                "\(squatter.name, privacy: .public): claimed name held by \(squatter.found.description, privacy: .public) — to be displaced"
            )
        }

        // Legal per the frontmatter table, meaningless at board level — ignore and log, never
        // tombstone, and **never migrate**: "a `deleted:` key at board level remains meaningless
        // — ignored and logged, preserved verbatim" (01-storage-format.md § Deletion). It is
        // deliberately absent from `legacyTombstones`: there is no item to relocate and no key
        // the app has any business removing from a file it was told to leave alone.
        if !boardDocument.deleted.isMissing {
            warn(.boardLevelDeletedIgnored)
        }

        // Every lane the walk read, in folder order — **not** `Lane` values yet. The board-wide
        // identity dedupe below decides which folders render at all, and a `Lane` is built only on
        // the far side of that decision, because a `Lane` carrying a withheld card would be exactly
        // the snapshot the invariant forbids.
        var walkedLanes: [WalkedLane] = []
        for laneURL in try directoryCandidates(in: boardRoot) {
            let laneName = laneURL.lastPathComponent
            // The app-claimed board-root names are not strays and must not warn as such. Hidden
            // entries never reach here anyway (`.trash` included), so this is the rule stated
            // rather than the mechanism relied on.
            guard !reservedRootNames.contains(laneName.lowercased()) else { continue }
            guard isUUIDShaped(laneName) else {
                warn(.nonUUIDFolderIgnored(path: laneName))
                continue
            }
            guard hasIndex(laneURL) else {
                warn(.missingIndex(path: laneName))
                continue
            }

            let lanePath = laneName + "/" + indexFileName
            let laneDocument = try readDocument(at: laneURL.appendingPathComponent(indexFileName), path: lanePath)
            let laneSchema = try validatedSchema(in: laneDocument, path: lanePath)
            let laneOrder = try validatedOrder(in: laneDocument, path: lanePath)

            var cards: [Card] = []
            for cardURL in try directoryCandidates(in: laneURL) {
                let cardName = cardURL.lastPathComponent
                let cardRelPath = laneName + "/" + cardName
                guard isUUIDShaped(cardName) else {
                    warn(.nonUUIDFolderIgnored(path: cardRelPath))
                    continue
                }
                guard hasIndex(cardURL) else {
                    warn(.missingIndex(path: cardRelPath))
                    continue
                }

                let card = try parseCard(at: cardURL, path: cardRelPath)

                // Noticed, never acted on: the relocation is the store's, through the Writer.
                let loose = looseFileNames(in: cardURL)
                if !loose.isEmpty {
                    defects.append(.looseCardFiles(LooseCardFiles(
                        laneID: ItemID(rawValue: laneName),
                        cardID: ItemID(rawValue: cardName),
                        title: card.title.value,
                        fileNames: loose
                    )))
                    logger.info("\(cardRelPath, privacy: .public): \(loose.count, privacy: .public) loose file(s) beside index.md — to be relocated into attachments/")
                }

                // Detection only, the loose-file precedent exactly: the relocation into `.trash/`
                // and the key's removal are the store's, through the Writer.
                if card.isDeleted {
                    defects.append(.legacyTombstone(LegacyTombstone(
                        kind: .card,
                        laneID: ItemID(rawValue: laneName),
                        cardID: ItemID(rawValue: cardName),
                        title: card.title.value
                    )))
                    logger.info("\(cardRelPath, privacy: .public): legacy 'deleted' key — card to be relocated into \(trashFolderName, privacy: .public)/")
                }

                cards.append(card)
            }

            if !laneDocument.deleted.isMissing {
                defects.append(.legacyTombstone(LegacyTombstone(
                    kind: .lane,
                    laneID: ItemID(rawValue: laneName),
                    cardID: nil,
                    title: laneDocument.title.value
                )))
                logger.info("\(laneName, privacy: .public): legacy 'deleted' key — lane to be returned live with the key removed")
            }

            walkedLanes.append(WalkedLane(
                name: laneName,
                schema: laneSchema,
                order: laneOrder,
                document: laneDocument,
                cards: Ranks.sortedForDisplay(cards, order: \.order, name: { $0.id.rawValue })
            ))
        }

        var trash: [Card] = []
        var trashKinds: [ItemID: IntegrityRules.ObjectKind] = [:]
        for cardURL in trashCandidates(in: boardRoot) {
            let cardName = cardURL.lastPathComponent
            let cardRelPath = trashFolderName + "/" + cardName
            guard isUUIDShaped(cardName) else {
                warn(.nonUUIDFolderIgnored(path: cardRelPath))
                continue
            }
            guard hasIndex(cardURL) else {
                warn(.missingIndex(path: cardRelPath))
                continue
            }
            let entry = try parseCard(at: cardURL, path: cardRelPath)
            // **The trash's discriminator, applied where the flat container needs it**
            // (01-storage-format.md § Deletion, re-ruled 2026-07-29): the *value* is trusted
            // outright, and only an unrecognized value or no key at all falls through to shape.
            // Reading the shape half is one directory listing, and only when the value did not
            // answer — see `looksLikeALaneFolder(_:)`.
            trashKinds[entry.id] = IntegrityRules.trashKind(
                kindValue: entry.document.kind.value,
                hasIdentityShapedChildIndex: looksLikeALaneFolder(cardURL)
            )
            trash.append(entry)
        }

        // **The board-wide identity dedupe** (01-storage-format.md § Fractal layout ▸ Rules:
        // "Duplicate ids within a board are never tolerated … the loader keeps exactly one occurrence
        // per id, board-wide"). Live lanes *and* `.trash/`, because board-wide means both containers —
        // and a snapshot carrying two items with equal ids is the one thing SwiftUI's `ForEach` does
        // not tolerate, which is why this is subtractive rather than advisory.
        //
        // Display order is settled here rather than in the `BoardModel` call below, because the
        // dedupe's last tie-break *is* traversal order and the rule needs the occurrences in it.
        let orderedLanes = Ranks.sortedForDisplay(walkedLanes, order: \.order, name: \.name)
        let orderedTrash = Ranks.sortedForDisplay(trash, order: \.order, name: { $0.id.rawValue })
        let verdict = dedupeIdentities(
            inBoardAt: boardRoot,
            lanes: orderedLanes,
            trash: orderedTrash,
            historyRanker: historyRanker
        )

        // Case twins are the **tolerate** tier: a spelling artifact of the item that rendered, with
        // nothing to do about it (§ Fractal layout ▸ Rules). They warn, exactly like every other
        // skipped stray, and are deliberately absent from `defects`.
        for twin in verdict.caseTwins {
            warn(.caseTwinIgnored(path: twin.path, winner: twin.winner))
        }
        // Content duplicates are **work**: withheld here, reminted by the store's scheduled heal
        // (re-ruled 2026-07-29 — the loader itself still never writes).
        for duplicate in verdict.duplicates {
            defects.append(.duplicateIdentity(duplicate))
            logger.warning(
                "\(duplicate.path, privacy: .public): duplicate id, withheld in favour of \(duplicate.winner, privacy: .public) — to be reminted"
            )
        }

        let withheld = Set(verdict.duplicates.map(\.path) + verdict.caseTwins.map(\.path))

        let model = BoardModel(
            rootURL: boardRoot,
            schema: boardSchema,
            title: boardDocument.title,
            created: boardDocument.created,
            modified: boardDocument.modified,
            modifiedBy: boardDocument.modifiedBy,
            deleted: boardDocument.deleted,
            background: boardDocument.background,
            icon: boardDocument.icon,
            iconColor: boardDocument.iconColor,
            template: boardDocument.value(for: templateKey),
            lanes: orderedLanes.compactMap { $0.rendered(withholding: withheld) },
            trash: orderedTrash.filter { !withheld.contains(trashFolderName + "/" + $0.id.rawValue) },
            document: boardDocument
        )

        return LoadResult(
            model: model,
            warnings: warnings,
            defects: defects,
            // Keyed by identity, so a withheld entry's reading has to go with it: two folders sharing
            // an id would otherwise leave a `kind` answer standing for the *other* one — the exact
            // ambiguity the dedupe exists to remove.
            trashKinds: trashKinds.filter { !withheld.contains(trashFolderName + "/" + $0.key.rawValue) }
        )
    }

    // MARK: - The board-wide identity dedupe

    /// Every identity-bearing folder in the board, deduped — the loader's half of the rule whose
    /// *deciding* is `IntegrityRules.dedupe(_:)`.
    ///
    /// **The gate comes first, and it is why this costs nothing on a healthy board**: a pass over the
    /// names alone answers "does any identity appear twice at all", and on the overwhelmingly common
    /// answer — no — nothing further happens: no `creationDateKey` reads, no history probes, no
    /// grouping. The disk reads below are paid for only by a board that actually has a collision.
    ///
    /// **Withheld subtrees are still walked** (settled — the import boundary's finest-grain rule read
    /// for the heal): a withheld *lane*'s cards are ordinary depth-2 occurrences and enter the
    /// inventory like any other, so a collision nested inside a losing lane is its own withheld
    /// occurrence with its own remint. Nothing deeper is an occurrence at all — "level is position",
    /// and a UUID-shaped folder under a card is content.
    ///
    /// **The container side is told to the rule, not derived from the path**: this walk knows which
    /// container it is in, and the container boundary is the rule's *first* tie-break (01-storage-format.md
    /// § Fractal layout ▸ Rules, stated 2026-07-29 — a live occurrence keeps the identity regardless of
    /// age), so re-deriving it from a `".trash/"` prefix downstream would be parsing back a fact that
    /// was in hand here.
    ///
    /// Occurrence order is the traversal order the rule's *last* tie-break is stated in: lanes in
    /// display order, each lane immediately followed by its cards in display order, then `.trash/`'s
    /// flat entries in display order. The trash sits last only because the walk meets it last —
    /// nothing rides on that any more, because the straddle case is decided by rung 0 long before
    /// traversal order is consulted.
    private static func dedupeIdentities(
        inBoardAt root: URL,
        lanes: [WalkedLane],
        trash: [Card],
        historyRanker: IdentityHistoryRanker?
    ) -> IntegrityRules.DedupeVerdict {
        typealias Container = IntegrityRules.IdentityOccurrence.Container
        // (path, name, container, title) in traversal order — the inventory, before anything is read
        // from disk.
        var inventory: [(path: String, name: String, container: Container, title: String?)] = []
        for lane in lanes {
            inventory.append((
                path: lane.name,
                name: lane.name,
                container: .live,
                title: lane.document.title.value
            ))
            for card in lane.cards {
                inventory.append((
                    path: lane.name + "/" + card.id.rawValue,
                    name: card.id.rawValue,
                    container: .live,
                    title: card.title.value
                ))
            }
        }
        for entry in trash {
            inventory.append((
                path: trashFolderName + "/" + entry.id.rawValue,
                name: entry.id.rawValue,
                container: .trashed,
                title: entry.title.value
            ))
        }

        // The gate.
        var seen: Set<String> = []
        var collides = false
        for entry in inventory where !seen.insert(IntegrityRules.canonicalIdentity(entry.name)).inserted {
            collides = true
            break
        }
        guard collides else { return IntegrityRules.DedupeVerdict(duplicates: [], caseTwins: []) }

        return IntegrityRules.dedupe(inventory.map { entry in
            IntegrityRules.IdentityOccurrence(
                path: entry.path,
                name: entry.name,
                container: entry.container,
                title: entry.title,
                birth: birthDate(of: root.appendingPathComponent(entry.path, isDirectory: true)),
                historyRank: historyRanker?.rank(entry.path)
            )
        })
    }

    /// A folder's filesystem birth date, `nil` when the volume does not keep one or the read fails —
    /// the second rung of the earlier-occurrence-wins ladder (§ Fractal layout ▸ Rules: "without
    /// history, the older folder (filesystem birth date) wins").
    ///
    /// `nil` is a first-class answer rather than a fallback date: an unreadable birth date must drop
    /// the comparison to traversal order, and substituting `.distantPast` here would silently make an
    /// unreadable folder *win* every comparison it entered.
    private static func birthDate(of folder: URL) -> Date? {
        (try? folder.resourceValues(forKeys: [.creationDateKey]))?.creationDate
    }

    /// One lane as the walk read it, before the dedupe decided what renders — a `Lane` minus the
    /// decision, which is the only reason it exists rather than the walk building `Lane` values
    /// directly.
    ///
    /// It carries the folder *name* rather than an `ItemID` because the dedupe's whole subject is
    /// spelling and identity being different questions, and the verbatim name is what answers both.
    private struct WalkedLane {
        let name: String
        let schema: Int
        let order: Double
        let document: FrontmatterDocument
        /// Already in display order — the traversal the dedupe's last tie-break is stated in.
        let cards: [Card]

        /// The `Lane` this becomes, or `nil` when the lane folder itself is withheld — a withheld
        /// lane takes its subtree out of the snapshot with it, and its cards' own collisions were
        /// already decided (they are occurrences in their own right).
        func rendered(withholding withheld: Set<String>) -> Lane? {
            guard !withheld.contains(name) else { return nil }
            return Lane(
                id: ItemID(rawValue: name),
                schema: schema,
                title: document.title,
                created: document.created,
                modified: document.modified,
                modifiedBy: document.modifiedBy,
                deleted: document.deleted,
                background: document.background,
                icon: document.icon,
                iconColor: document.iconColor,
                order: order,
                width: document.width,
                cards: cards.filter { !withheld.contains(name + "/" + $0.id.rawValue) },
                document: document
            )
        }
    }

    // MARK: - The earlier-occurrence-wins history seam

    /// **Where git path history plugs into the duplicate-id winner rule** (01-storage-format.md
    /// § Fractal layout ▸ Rules: "on git boards, the path history already tracks outranks the
    /// newcomer (both tracked: the path that entered history first)").
    ///
    /// A seam rather than an implementation because the first rung of that ladder is unbuildable in
    /// base: base Lanework links no git machinery at all (12-editions.md; `scripts/verify-editions.sh`
    /// scans the base binary to prove it), so the loader consults an injected ranker and falls through
    /// to birth date and traversal order when there is none — which is every base board, and every Pro
    /// board without a repo.
    ///
    /// Deliberately one closure and no protocol: the loader asks one question — "how early did this
    /// path enter history" — and pro-m1's implementation answers it from `git log --diff-filter=A
    /// --follow`-shaped plumbing behind the edition seam. `nil` means "untracked, or no history
    /// here", which the rule reads as *outranked by anything tracked*.
    ///
    /// - Parameter rank: keyed by the occurrence's **board-root-relative path**, which is what a
    ///   repo's path history knows; lower is earlier.
    public struct IdentityHistoryRanker: Sendable {
        public let rank: @Sendable (String) -> Int?

        public init(rank: @escaping @Sendable (String) -> Int?) {
            self.rank = rank
        }
    }

    /// Whether a folder in `.trash/` has the *shape* of a lane — at least one identity-shaped child
    /// holding its own `index.md` (01-storage-format.md § Deletion: "UUID-shaped children with
    /// their own `index.md` → lane … else card").
    ///
    /// Only reached when `kind` did not answer (`IntegrityRules.trashKind`'s `@autoclosure`), and
    /// deliberately not a parse: this asks what the folder *looks like*, not whether anything inside
    /// it would load. A trashed lane's cards are never enumerated as levels — the walk stops at a
    /// trash entry exactly as it stops at a card under a lane.
    private static func looksLikeALaneFolder(_ folder: URL) -> Bool {
        let children = (try? directoryCandidates(in: folder)) ?? []
        return children.contains { isUUIDShaped($0.lastPathComponent) && hasIndex($0) }
    }

    /// One card folder read into a `Card` — **the card parse, shared by both containers**.
    ///
    /// A trashed card is "an ordinary card in a special place" (03-board-ui.md § Trash), and this
    /// function is what makes that literally true rather than a claim two code paths have to keep
    /// agreeing on: the same strict `schema`/`order` validation, the same attachment listing, the
    /// same verbatim document. Its callers keep what genuinely differs by container — the
    /// lane-keyed loose-file and legacy-tombstone channels — outside it.
    ///
    /// `path` is root-relative and names the *folder*; the errors this throws name its `index.md`.
    /// Callers guard `isUUIDShaped` and `hasIndex` first, exactly as the lane walk always has.
    private static func parseCard(at cardURL: URL, path: String) throws(BoardLoadError) -> Card {
        let cardPath = path + "/" + indexFileName
        let document = try readDocument(at: cardURL.appendingPathComponent(indexFileName), path: cardPath)
        let schema = try validatedSchema(in: document, path: cardPath)
        let order = try validatedOrder(in: document, path: cardPath)

        return Card(
            id: ItemID(rawValue: cardURL.lastPathComponent),
            schema: schema,
            title: document.title,
            created: document.created,
            modified: document.modified,
            modifiedBy: document.modifiedBy,
            deleted: document.deleted,
            background: document.background,
            icon: document.icon,
            iconColor: document.iconColor,
            order: order,
            attachments: attachmentNames(in: cardURL),
            document: document
        )
    }

    /// The candidate card folders inside `<root>/.trash/`, or `[]` when there is no trash.
    ///
    /// **Absent is empty, not an error** — the container is minted by the first delete, so most
    /// boards never have one, and a board without a trash is a board with an empty trash.
    ///
    /// **A `.trash` that is not a plain directory yields nothing**: a file by that name, or a
    /// *symlink* — "symlinks are never traversed" (01-storage-format.md § Fractal layout ▸ Rules),
    /// and a symlinked trash would render bytes living outside the board that FSEvents never
    /// reports. Not a `LoadWarning`: that is the stray vocabulary, and a claimed name is not a
    /// stray. Since 2026-07-29 it is not merely logged either — the walk reports it as a
    /// `Defect.claimedNameSquatted` (detected up in `load`, before the lanes) and a scheduled heal
    /// displaces it. Until that heal lands the loader keeps this empty-trash read, which is exactly
    /// the "window measured in one reload, not a standing state" the ruling accepts.
    ///
    /// Entries are `directoryCandidates` — hidden entries and symlinks already excluded, in
    /// folder-name order — so the trash gets the same stray tolerance every other container gets,
    /// including the one that matters most here: a **lane-shaped nesting** inside `.trash` (a
    /// whole lane folder dropped in by hand) is not a level. Its own UUID-shaped children are
    /// never enumerated, because the walk stops at a card exactly as it does under a lane; the
    /// outer folder either parses as a card (it has an `index.md`) or is skipped as
    /// `.missingIndex`, and either way nothing below it renders.
    private static func trashCandidates(in boardRoot: URL) -> [URL] {
        let trashURL = boardRoot.appendingPathComponent(trashFolderName, isDirectory: true)
        guard let values = try? trashURL.resourceValues(forKeys: [.isDirectoryKey, .isSymbolicLinkKey]) else {
            return []
        }
        guard values.isDirectory == true, values.isSymbolicLink != true else {
            logger.warning("\(trashFolderName, privacy: .public): not a plain directory, treated as an empty trash until the heal displaces it")
            return []
        }
        return (try? directoryCandidates(in: trashURL)) ?? []
    }

    // MARK: - Filesystem helpers

    private static func checkIsReadableDirectory(_ url: URL) throws(BoardLoadError) {
        var isDirectory: ObjCBool = false
        guard FileManager.default.fileExists(atPath: url.path, isDirectory: &isDirectory) else {
            throw BoardLoadError(path: ".", reason: .unreadableRoot(message: "no such file or directory"))
        }
        guard isDirectory.boolValue else {
            throw BoardLoadError(path: ".", reason: .notADirectory)
        }
    }

    private static func hasIndex(_ folder: URL) -> Bool {
        FileManager.default.fileExists(atPath: folder.appendingPathComponent(indexFileName).path)
    }

    /// The names of `<card>/attachments/`'s **top-level regular files** — the flat view
    /// 01-storage-format.md § Attachments specifies ("top-level files only"; "subfolders are
    /// tolerated, preserved verbatim … and not surfaced"). `[]` when there is no `attachments/`.
    ///
    /// Three exclusions, the same three `directoryCandidates` makes and for the same reasons:
    /// hidden entries (`.DS_Store` and friends are not the user's attachments), directories (a
    /// subfolder stays reachable through Reveal in Finder and through body-relative paths, but
    /// never appears as an attachment), and symlinks (this loader resolves nothing — the same
    /// stance the level walk takes).
    ///
    /// **Finder order** (`localizedStandardCompare`), so `"shot 2.png"` sorts before
    /// `"shot 10.png"`: the order has to be stable across loads, and where it is already the
    /// sidebar's order it may as well be the same one.
    ///
    /// Failure is silent: an unlistable directory yields `[]`. Fail-fast is reserved for
    /// structure (01-storage-format.md § Malformed input), and this field decorates a card — a
    /// permissions race here must never be the reason a whole board refuses to open.
    ///
    /// Internal rather than `private`: `BoardWriter.listAttachments` — the card window sidebar's
    /// authoritative listing — answers through this same function behind its own card-folder
    /// guard, so the face and the sidebar can never disagree about what a card's attachments are.
    /// It is also why the folder name is read off `BoardWriter`, which owns it as the one folder
    /// the app ever creates under a card.
    static func attachmentNames(in cardFolder: URL) -> [String] {
        let folder = cardFolder.appendingPathComponent(
            BoardWriter.attachmentsFolderName, isDirectory: true
        )
        guard let entries = try? FileManager.default.contentsOfDirectory(
            at: folder,
            includingPropertiesForKeys: [.isRegularFileKey, .isSymbolicLinkKey],
            options: [.skipsHiddenFiles]
        ) else {
            return []
        }

        return entries
            .filter { url in
                guard let values = try? url.resourceValues(forKeys: [.isRegularFileKey, .isSymbolicLinkKey]) else {
                    return false
                }
                return values.isRegularFile == true && values.isSymbolicLink != true
            }
            .map(\.lastPathComponent)
            .sorted { $0.localizedStandardCompare($1) == .orderedAscending }
    }

    /// A card folder's **loose top-level files** — the one carve-out to uniform stray tolerance
    /// (01-storage-format.md § Fractal layout ▸ Rules, settled 2026-07-28, "Lanework-owns-the-board"):
    /// "a regular file sitting beside a card's `index.md` (not `attachments/`, not a reserved name)
    /// belongs in `attachments/`, and the app moves it there".
    ///
    /// **This function only notices.** It opens nothing, moves nothing, and creates nothing; the
    /// relocation is `BoardWriter.relocateLooseFiles`, run through the store's write bracket. A load
    /// is a pure function of the tree and stays one.
    ///
    /// Four exclusions, three of them `attachmentNames(in:)`' own and for its reasons:
    ///
    /// - **Directories.** The carve-out is exactly *files*. A stray folder in a card — a nested
    ///   clone, a hand-made subfolder — keeps the verbatim posture, because "relocating a directory
    ///   into the flat attachment model would be wrong".
    /// - **Symlinks**, never touched and never traversed (§ Rules) — the same stance the level walk
    ///   takes. `isSymbolicLink` is checked *beside* `isRegularFile` rather than trusted to imply
    ///   it, exactly as `directoryCandidates` does, so a link pointing at a file is excluded on its
    ///   own account.
    /// - **Hidden entries.** `.DS_Store` and friends are not the user's files, and relocating one
    ///   would surface it in a card's attachment list — the loudest possible way to be wrong about
    ///   a file nobody wrote on purpose. It is also what keeps a crashed write's dot-prefixed
    ///   residue out of the relocation.
    /// - **The reserved card-level names** (`reservedCardChildNames`), case-insensitively.
    ///
    /// Finder order (`localizedStandardCompare`), like every other name listing here, so the notice
    /// the store posts names files the way the board would sort them.
    ///
    /// Failure is silent (`[]`): a permissions race here must never be the reason a board refuses
    /// to open, and "nothing to relocate" is the safe reading of "cannot tell".
    static func looseFileNames(in cardFolder: URL) -> [String] {
        guard let entries = try? FileManager.default.contentsOfDirectory(
            at: cardFolder,
            includingPropertiesForKeys: [.isRegularFileKey, .isSymbolicLinkKey],
            options: [.skipsHiddenFiles]
        ) else {
            return []
        }

        return entries
            .filter { url in
                guard !reservedCardChildNames.contains(url.lastPathComponent.lowercased()),
                      let values = try? url.resourceValues(forKeys: [.isRegularFileKey, .isSymbolicLinkKey])
                else {
                    return false
                }
                return values.isRegularFile == true && values.isSymbolicLink != true
            }
            .map(\.lastPathComponent)
            .sorted { $0.localizedStandardCompare($1) == .orderedAscending }
    }

    /// Whether `name` has a UUID's shape — hex, `8-4-4-4-12`, **any case and any version** —
    /// gating lane/card level detection (01-storage-format.md § Fractal layout ▸ Rules, "Name
    /// shape gates level detection"). This is *the* identity predicate, and it is deliberately
    /// **shape-only**: lowercase v4 is the app's emission rule, not the gate.
    ///
    /// - **Any case.** `uuidgen(1)` and Swift's own `UUID().uuidString` both print *uppercase*,
    ///   so a strict lowercase gate would turn an agent's standard-tool card into a silently
    ///   skipped stray — the worst failure mode for a files-first app. Accept liberally, emit
    ///   conservatively: `BoardWriter` still writes only lowercase v4 and never renames an
    ///   existing folder to canonicalize it.
    /// - **Any version.** The version (13th hex digit) and variant (17th hex digit) nibbles are
    ///   **not** validated: they protect no invariant here — an agent's v7 is exactly as unique
    ///   as a v4 — and the loader's job is recognizing the folder-naming *convention*, not
    ///   re-deriving RFC 4122 conformance every load.
    ///
    /// Equivalent to "does `UUID(uuidString:)` parse it", which is how 01-storage-format.md
    /// states the rule; kept as a manual scan because that is the cheaper answer on the hot path
    /// (every folder of every load) and needs no bridging.
    ///
    /// Recognizing a name is not the same as *comparing* two of them: identity comparison is
    /// UUID-*value* equality, so two case-spellings of one UUID are one identity everywhere —
    /// see `ItemID` (`BoardModel.swift`), which stores the folder's exact spelling but compares
    /// canonically.
    ///
    /// Internal rather than `private`: `BoardWriter.renumberVisibleChildren` walks the same
    /// candidates the loader walked, and level detection has to be one rule, not two.
    ///
    /// **The rule itself is `IntegrityRules.isIdentityShaped`** (settled 2026-07-29 — the one
    /// vocabulary of object validity). This is the loader's spelling of it and nothing more: one
    /// predicate, one implementation, no parallel derivation.
    static func isUUIDShaped(_ name: String) -> Bool {
        IntegrityRules.isIdentityShaped(name)
    }

    /// Direct subdirectories of `folder`, in deterministic (folder-name) order, excluding
    /// hidden entries (`.DS_Store`, `.git`, …) and symlinks — the loader's uniform stray
    /// tolerance (01-storage-format.md § Fractal layout ▸ Rules). Stray *files* are excluded
    /// here too: only directories are level candidates at all, and the caller further narrows
    /// those to actual lane/card candidates by name shape (`isUUIDShaped`) before doing
    /// anything else with them.
    ///
    /// **`.trash/` is hidden, so it never appears among a board root's candidates** — which is
    /// exactly right: it is a container, not a level, and the walk reaches it by name instead
    /// (`trashCandidates(in:)`). This same call then enumerates *inside* it, so the trash's own
    /// children get identical stray tolerance.
    ///
    /// An unreadable non-root folder (permission changed mid-walk, races) degrades to "no
    /// candidates" rather than failing the whole load — fail-fast is reserved for the board
    /// root and for malformed `index.md` content, not transient directory-listing races below
    /// it.
    /// Internal rather than `private`: `BoardWriter.renumberVisibleChildren` enumerates
    /// siblings through this same door, so the writer's idea of "the children" can never drift
    /// from the loader's. It is also why `BoardWriter`'s temp files are dot-prefixed — the
    /// `.skipsHiddenFiles` here is what makes a crashed write's residue invisible to a load.
    static func directoryCandidates(in folder: URL) throws(BoardLoadError) -> [URL] {
        guard let entries = try? FileManager.default.contentsOfDirectory(
            at: folder,
            includingPropertiesForKeys: [.isDirectoryKey, .isSymbolicLinkKey],
            options: [.skipsHiddenFiles]
        ) else {
            return []
        }

        return entries
            .filter { url in
                guard let values = try? url.resourceValues(forKeys: [.isDirectoryKey, .isSymbolicLinkKey]) else {
                    return false
                }
                return values.isDirectory == true && values.isSymbolicLink != true
            }
            .sorted { $0.lastPathComponent < $1.lastPathComponent }
    }

    // MARK: - Document reading + field validation

    /// **Strict, byte-faithful UTF-8** — the same decode `BoardWriter` uses, and for the same
    /// reason: Foundation's NSString-backed `String(contentsOf:encoding:)` silently strips a
    /// leading BOM, which would let a BOM'd file *load* here and then refuse every write over
    /// in `BoardWriter` — a baffling split. 01-storage-format.md § Fractal layout ▸ Rules is
    /// explicit that a BOM'd file is rejected at load (it fails the frontmatter delimiter);
    /// decoding byte-faithfully is what makes that stated rejection actually happen.
    private static func readDocument(at url: URL, path: String) throws(BoardLoadError) -> FrontmatterDocument {
        let data: Data
        do {
            data = try Data(contentsOf: url)
        } catch {
            throw BoardLoadError(
                path: path,
                reason: .unparseableYAML(message: "could not read file: \(error.localizedDescription)", line: nil)
            )
        }
        return try parseDocument(data, path: path)
    }

    /// The decode-and-parse half of `readDocument(at:path:)`, over bytes rather than a URL.
    ///
    /// Split out for the raw-source outlet, which validates bytes that are **not on disk yet**
    /// (`validateCardIndex`) — and split rather than copied on purpose: "Apply validates through the
    /// same fail-fast parse the loader uses" (05-card-window.md ▸ Raw source outlet) is only true if
    /// it is literally the same function. The strict UTF-8 decode is half of what that buys — a BOM'd
    /// or non-UTF-8 proposal is rejected here by the same two lines that reject one on disk
    /// (01-storage-format.md § Fractal layout ▸ Rules).
    static func parseDocument(_ data: Data, path: String) throws(BoardLoadError) -> FrontmatterDocument {
        guard let text = String(validating: data, as: UTF8.self) else {
            throw BoardLoadError(path: path, reason: .unparseableYAML(message: "file is not UTF-8", line: nil))
        }

        do {
            return try FrontmatterDocument.parse(text)
        } catch {
            let line: Int? = if case let .unparseableYAML(_, line) = error { line } else { nil }
            throw BoardLoadError(path: path, reason: .unparseableYAML(message: error.description, line: line))
        }
    }

    /// Whether `data` would load as a **card's** `index.md` — the raw-source Apply's gate
    /// (05-card-window.md ▸ Raw source outlet: "Apply validates through the same fail-fast parse the
    /// loader uses (detailed alert on error, stays in source mode) before writing byte-for-byte").
    ///
    /// **Exactly the three checks `load(boardRoot:)` runs on a card**, in its order and through its
    /// own functions: decode + parse (`parseDocument`), then `schema` (present, well-formed, not
    /// newer than this app) and `order` (present, well-formed) — the two fields a card must carry.
    /// Nothing card-shaped is checked beyond that, because nothing else *is*: `title` is optional,
    /// unknown keys are the whole point of the outlet, and the body is free text.
    ///
    /// It deliberately does **not** check `uneditableShape`: that refusal exists for surgical
    /// span edits (`BoardWriter.updateIndex`), and raw source replaces the whole file — a flow-mapping
    /// frontmatter is precisely one of the things the escape hatch exists to let a user rewrite.
    ///
    /// The error is the loader's own, undiluted, so the alert can show the taxonomy's display text
    /// (line numbers included) rather than a re-worded copy.
    ///
    /// - Parameter path: what the error names — `indexFileName` from every call site today, which is
    ///   what the card window's alert is about.
    ///
    /// **The rule is `IntegrityRules.validateIndex(_:path:kind:supportedSchema:)`**, generalized per
    /// kind (02-architecture.md ▸ Components). This spelling stays because it is what the card
    /// window asks — "would this load as a card?" — and because pinning the kind at the call site is
    /// what keeps the outlet's gate from drifting when a second kind gains one.
    public static func validateCardIndex(_ data: Data, path: String) throws(BoardLoadError) -> FrontmatterDocument {
        try IntegrityRules.validateIndex(data, path: path, kind: .card, supportedSchema: supportedSchema)
    }

    /// The per-field validators are `IntegrityRules`' — the rulebook (02-architecture.md ▸
    /// Components). These two forward so the walk above reads as it always did.
    private static func validatedSchema(in document: FrontmatterDocument, path: String) throws(BoardLoadError) -> Int {
        try IntegrityRules.validatedSchema(in: document, path: path, supportedSchema: supportedSchema)
    }

    private static func validatedOrder(in document: FrontmatterDocument, path: String) throws(BoardLoadError) -> Double {
        try IntegrityRules.validatedOrder(in: document, path: path)
    }
}

// MARK: - Result

/// A successful load: the snapshot, anything tolerated-but-notable encountered along the way, and
/// the pending work the walk found. `warnings` is also logged as it accumulates
/// (`os.Logger(subsystem: "dev.rzen.indie.Kanban", category: "loader")`) so it shows up in Console
/// even if a caller never inspects it.
public struct LoadResult: Sendable {
    public var model: BoardModel
    public var warnings: [LoadWarning]

    /// **The typed defect stream** — everything this walk found that is pending *work*
    /// (02-architecture.md ▸ Components ▸ IntegrityRules, settled 2026-07-29). One channel, not
    /// three: loose card files, legacy `deleted:` keys, and a claimed board-root name held by the
    /// wrong kind of node all classify as `IntegrityRules.Defect`, carry their own signature, and
    /// are healed by the one engine (`HealScheduler`).
    ///
    /// **Deliberately not `warnings`**, which stays the *tolerate*-tier vocabulary: "this was
    /// ignored, it is staying exactly where it is, there is nothing to do". A defect says the
    /// opposite — it is work, and the store acts on it. Folding the two would also throw away
    /// everything a heal needs (which lane, which card, which title, which names) and force it to be
    /// re-derived from a display string.
    ///
    /// Nothing renders this. Order is the walk's: the board root's claimed names, then, lane by
    /// lane, each lane's cards and then the lane itself.
    ///
    /// **Cards in `.trash/` are deliberately not walked for loose files in this version.** The
    /// loose-file defect is keyed by lane (`LooseCardFiles.laneID`, the store's path key) and a
    /// trashed card has no lane; widening the key is the store-side change that belongs with the
    /// store-side scheduling. Loose files beside a trashed card's `index.md` therefore keep the
    /// ordinary stray posture — tolerated, preserved verbatim — and are tidied the moment the card
    /// is restored into a lane, which is the only state in which they matter.
    ///
    /// Board-level `deleted:` never appears here — it is meaningless, ignored and logged
    /// (`LoadWarning.boardLevelDeletedIgnored`), and nothing about it is the app's to rewrite.
    public var defects: [IntegrityRules.Defect] = []

    /// What each `.trash/` entry **is**, by the trash's own discriminator
    /// (`IntegrityRules.trashKind`; 01-storage-format.md § Deletion, re-ruled 2026-07-29): the
    /// `kind` value trusted outright, falling through to shape only when it does not answer.
    ///
    /// A *reading*, not a rendering: `BoardModel.trash` parses every entry through the one card
    /// parse (a trashed card is "an ordinary card in a special place"), and the container is flat,
    /// so this is where the answer to "which of these was a lane?" lives until the lanes-in-trash
    /// surface consumes it. Keyed by identity, so it survives the display sort.
    public var trashKinds: [ItemID: IntegrityRules.ObjectKind] = [:]

    /// The cards this walk found holding loose files — a **view over `defects`**, under the name it
    /// has always had (01-storage-format.md § Fractal layout ▸ Rules, settled 2026-07-28).
    ///
    /// In the order the walk met them, which is the order the relocation writes them in.
    public var looseCardFiles: [LooseCardFiles] {
        defects.compactMap { if case let .looseCardFiles(work) = $0 { work } else { nil } }
    }

    /// The legacy `deleted:` keys this walk found — the retired tombstone model's migration input,
    /// as a **view over `defects`** (01-storage-format.md § Deletion).
    ///
    /// Order is the walk's: a lane's tombstoned cards, then the lane itself, lane by lane.
    public var legacyTombstones: [LegacyTombstone] {
        defects.compactMap { if case let .legacyTombstone(work) = $0 { work } else { nil } }
    }

    /// The claimed board-root name found held by the wrong kind of node, if any — a **view over
    /// `defects`** (01-storage-format.md § Fractal layout ▸ Rules, ruled 2026-07-29).
    public var claimedNameSquatters: [ClaimedNameSquatter] {
        defects.compactMap { if case let .claimedNameSquatted(work) = $0 { work } else { nil } }
    }

    /// The later occurrences this walk withheld from `model` — a **view over `defects`**
    /// (01-storage-format.md § Fractal layout ▸ Rules; re-ruled 2026-07-29 — the silent remint).
    ///
    /// **The one defect the snapshot is already missing.** Every other defect names something that is
    /// both on disk and in the model; a withheld duplicate is on disk and deliberately *not* in the
    /// model, which is what makes the one-item-per-id invariant hold by construction rather than by
    /// hope. In traversal order — which is the order the remint writes them and the notice names them.
    public var duplicateIdentities: [DuplicateIdentity] {
        defects.compactMap { if case let .duplicateIdentity(work) = $0 { work } else { nil } }
    }
}

/// A tolerated anomaly the loader kept going past. Never blocks a load — see `BoardLoadError`
/// for what does.
public enum LoadWarning: Sendable, Equatable, CustomStringConvertible {
    /// A **UUID-shaped** folder below the board root has no `index.md` — skipped, not
    /// fail-fast (an interrupted two-step create must not brick the board). `path` is relative
    /// to the board root. Only reachable for a folder that passed `isUUIDShaped`; a
    /// non-UUID-shaped folder missing `index.md` gets `.nonUUIDFolderIgnored` instead, never
    /// this case.
    case missingIndex(path: String)

    /// A lane/card-depth folder whose name doesn't have a UUID's shape (`isUUIDShaped` — hex,
    /// `8-4-4-4-12`, any case, any version) —
    /// skipped, not fail-fast, regardless of whether it holds a valid `index.md`, a broken one,
    /// or none at all (01-storage-format.md § Fractal layout ▸ Rules, "Name shape gates level
    /// detection"). Preserved verbatim on disk, never descended into. `path` is relative to the
    /// board root.
    case nonUUIDFolderIgnored(path: String)

    /// A board-level `deleted:` key is legal per the frontmatter table but meaningless
    /// (01-storage-format.md § Deletion) — ignored, never tombstones the board.
    case boardLevelDeletedIgnored

    /// A folder whose name is a **case-spelled twin** of another occurrence of the same identity —
    /// one item typed two ways (01-storage-format.md § Fractal layout ▸ Rules): a spelling some *live*
    /// occurrence carries wins over one only trash ghosts carry (the container preference, stated
    /// 2026-07-29), and among the candidates the canonical all-lowercase spelling wins where present,
    /// else the lexicographically first. This is the loser. Skipped, preserved verbatim, never
    /// rendered — **and never reminted**: it is a spelling artifact of the item that rendered, not a
    /// copy, so there is nothing to heal.
    ///
    /// Its home is here rather than in the defect stream precisely because of that: a warning says
    /// "this was ignored, it is staying exactly where it is, there is nothing to do", which is the
    /// whole of the tolerate tier's verdict on it. Both paths are root-relative.
    case caseTwinIgnored(path: String, winner: String)

    public var description: String {
        switch self {
        case let .missingIndex(path):
            "\(path): folder has no index.md, skipped"
        case let .nonUUIDFolderIgnored(path):
            "\(path): folder name is not UUID-shaped, ignored as a stray"
        case .boardLevelDeletedIgnored:
            "index.md: board-level 'deleted' key is meaningless, ignored"
        case let .caseTwinIgnored(path, winner):
            "\(path): case-spelled twin of \(winner), ignored as a spelling artifact"
        }
    }
}

// MARK: - Error

/// A fail-fast structural failure loading a board — loud and specific: `path` (relative to
/// the board root where one exists) plus `reason` says exactly what's wrong. No partial
/// loads: throwing this means `BoardLoader.load` produced nothing at all.
public struct BoardLoadError: Error, Sendable, Equatable, CustomStringConvertible {
    public let path: String
    public let reason: Reason

    public var description: String { "\(path): \(reason.description)" }

    public enum Reason: Sendable, Equatable, CustomStringConvertible {
        /// The board root itself has no `index.md` — unlike every level below it, this is not
        /// skip-and-warn: there is no board without one.
        case boardRootMissingIndex
        /// Wraps any `FrontmatterError` from parsing — bad delimiters, bad YAML, a
        /// frontmatter block that isn't a mapping. `line` is 1-based within the file when the
        /// underlying error carries one.
        case unparseableYAML(message: String, line: Int?)
        case missingSchema
        case malformedSchema(raw: String)
        /// `schema` is present, valid, and greater than this app's `supportedSchema`.
        case schemaNewerThanApp(found: Int)
        /// `order` is required on lanes and cards, never on the board itself.
        case missingOrder
        case malformedOrder(raw: String)
        /// The board root exists but is a file, not a directory.
        case notADirectory
        /// The board root doesn't exist, or its contents couldn't be listed.
        case unreadableRoot(message: String)

        public var description: String {
            switch self {
            case .boardRootMissingIndex:
                "board root is missing index.md"
            case let .unparseableYAML(message, line):
                if let line {
                    "unparseable YAML at line \(line): \(message)"
                } else {
                    "unparseable YAML: \(message)"
                }
            case .missingSchema:
                "missing required 'schema' field"
            case let .malformedSchema(raw):
                "malformed 'schema' field: \(raw)"
            case let .schemaNewerThanApp(found):
                "schema \(found) is newer than this app supports (schema \(BoardLoader.supportedSchema))"
            case .missingOrder:
                "missing required 'order' field"
            case let .malformedOrder(raw):
                "malformed 'order' field: \(raw)"
            case .notADirectory:
                "board root is not a directory"
            case let .unreadableRoot(message):
                "board root is unreadable: \(message)"
            }
        }
    }
}