Implement the hybrid clipboard with deferred cut

⌘X/⌘C/⌘V for cards and lanes per 04-interactions.md § Clipboard:

- ClipboardStore stages full folder snapshots eagerly at the gesture into
  Application Support (at most the current copy; sweep at launch and on
  each copy purges what the pasteboard no longer references; a copy made
  before quitting pastes whole after restart) and writes the pasteboard a
  JSON manifest — every entry embedding its index.md, lane entries their
  cards' too — plus plain-text titles.
- Cut is Finder-style deferred: items dim in place off pendingCut, void on
  pasteboard takeover (changeCount, no timers), source-board close, or
  per-item external tombstoning; the first armed paste moves the surviving
  originals whole (tombstoned interior cards land in the destination's
  trash), a second paste materializes copies from staging.
- Paste anchors by the shared flatten-order rule (NewCardTarget's anchor,
  extracted); a tombstoned selection never anchors; lane paste reaches the
  right end and stays enabled on a zero-lane board; paste into the source
  board is the within-board lane duplicate; copies keep created, take
  fresh GUIDs, and strip tombstoned cards; trash-sourced copies strip
  deleted: at materialization; ⌘X is disabled on the trash side.
- A degraded paste is loud, never silent: staging gone → the embedded
  index.md fallback lands content-intact, attachments absent, and a
  BannerCenter-phrased row names what was lost.
- The standard Edit items validate through conditionally-attached
  onCommand handlers, so AppKit's enablement mirrors the availability
  predicates; text fields keep their own clipboard while focused.

879 unit tests (68 new).

Claude-Session: https://claude.ai/code/session_01SR4XGjmBE16ZUYWpfFHXwY

Kanban/LiveStore/BannerCenter.swift

@@ -271,6 +271,43 @@ public final class BannerCenter {
         signposts.insert(InfoSignpost(message: message), at: 0)
     }
 
+    /// One item a degraded paste could not bring its attachments with — what
+    /// `degradedPasteMessage(for:)` names.
+    ///
+    /// `title` is the item's as written, `nil` for an untitled one: "Untitled" is a rendering, never
+    /// a value (03-board-ui.md § Card face), and the phrasing below says "the item" instead, exactly
+    /// as `actionPhrase(for:)` does for a failure whose title never got read.
+    public struct AttachmentLoss: Sendable, Equatable {
+        public let title: String?
+        public let attachments: Int
+
+        public init(title: String?, attachments: Int) {
+            self.title = title
+            self.attachments = attachments
+        }
+    }
+
+    /// **The degraded paste** (04-interactions.md ▸ Clipboard, settled): the staged snapshot was
+    /// missing or unreadable, so the paste fell back to the manifest's embedded `index.md` — content
+    /// intact, attachments absent — and this is the row that says so. "A degraded paste is loud,
+    /// never silent … the user never discovers an empty `attachments/` later."
+    ///
+    /// **A signpost, not a `oneShot`**, and the choice is the vocabulary's rather than a compromise:
+    /// 02-architecture.md's `oneShot` is *a write that did not happen*, carrying a `BoardWriteError`,
+    /// and nothing here failed — the items landed, whole but for files that were never on the
+    /// pasteboard's side of the transfer. A signpost is the other member of the same lifecycle class
+    /// ("one-shots dismiss"): it reports something that already happened, it has no timeout, and only
+    /// the user clears it, which is the whole of "never evaporates unread". What it costs is
+    /// precedence — a signpost ranks last and may collapse behind "+N more" — which is the one place
+    /// this row reads quieter than 04's "loud" deserves. See the report's design-gap note.
+    ///
+    /// An empty list posts nothing: a fallback that lost no attachments lost nothing at all, and a
+    /// banner announcing that would be noise.
+    public func postDegradedPaste(_ losses: [AttachmentLoss]) {
+        guard let message = Self.degradedPasteMessage(for: losses) else { return }
+        postSignpost(message)
+    }
+
     /// Removes a dismissable row: a one-shot failure or a signpost. **An id that names an
     /// in-progress operation is ignored** rather than ending it, because "dismiss" and "cancel" are
     /// different promises and a row that offers one must never quietly do the other.
@@ -512,6 +549,32 @@ public final class BannerCenter {
         return "\(subject): \(reason) — showing the last good view"
     }
 
+    /// The degraded paste's line — 04-interactions.md's own example sentence, "Pasted 'Fix login'
+    /// without its 3 attachments", generalized over the two axes it can vary on.
+    ///
+    /// **It names exactly what was lost**, which is what the design asks for and what decides every
+    /// choice below: the count is real (never "some"), the singular and the plural are both spelled,
+    /// and a multi-item paste totals the attachments rather than listing every title — a banner is one
+    /// line, and "2 items" plus the true total is the honest summary where a truncated list would not
+    /// be. `nil` for an empty list: nothing was lost, so there is nothing to say.
+    ///
+    /// The count is the item's `attachments/` as the snapshot listed it at copy time — the design's
+    /// own vocabulary for what a card carries (01-storage-format.md § Attachments). A stray file
+    /// sitting loose in the card folder is not in it and is not named here; see the report's
+    /// design-gap note.
+    public nonisolated static func degradedPasteMessage(for losses: [AttachmentLoss]) -> String? {
+        guard !losses.isEmpty else { return nil }
+        let total = losses.reduce(0) { $0 + $1.attachments }
+        guard total > 0 else { return nil }
+
+        guard losses.count == 1, let only = losses.first else {
+            return "Pasted \(losses.count) items without their \(total) attachments"
+        }
+        let subject = only.title.map { "'\($0)'" } ?? "the item"
+        let tail = total == 1 ? "its attachment" : "its \(total) attachments"
+        return "Pasted \(subject) without \(tail)"
+    }
+
     /// The suspended-history line. It names the *consequence* the user cares about — undo and the
     /// flush-before-overwrite guarantee are degraded — rather than the git mechanics, and carries
     /// the diagnosis as its tail.