Build Preview mode rendering

The card body's resting state: swift-markdown (pinned 0.8.0, smart
typography off — Preview renders the bytes on disk) parsed into a pure
BodyMarkup model with UTF-8 source offsets, rendered on one hosted
TextKit 1 NSTextView — chosen because find-in-text is NSTextFinder,
checkbox clicks reuse AppKit character hit-testing, links are .link
attributes, and NSTextTable's automatic layout is exactly the
columns-sized-to-contents rule. The GFM subset renders per 05; HTML
stays verbatim code-styled text; relative images resolve against the
card folder while remote URLs are never fetched, drawing a quiet chip
instead. Task checkboxes are live: a click flips exactly one byte
through a fresh-read, refuse-uneditable, stamp, atomic-replace write —
the app's only offset-addressed write, so a moved target refuses as
staleTarget and what the user saw decides the direction, netting one
toggle on a double-click. Empty bodies open in Edit per CardBodyMode's
opening rule, applied once; the Edit surface itself stays an honest
read-only stub until its card. FindCommand prefers the card body's
find over board search when a card window is focused.

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

Kanban/LiveStore/BannerCenter.swift

@@ -641,6 +641,11 @@ public final class BannerCenter {
             // failure the user did not provoke is exactly the one they have no other way to learn
             // about.
             "Couldn't move '\(filename)' into attachments"
+        case let .toggleTask(title):
+            // The user's word for it, not the file's: they ticked a box. The card is named where
+            // the read that preceded the flip learned its title, so a body write that refused says
+            // *which* card refused it — a card window is not always the frontmost thing on screen.
+            if let title { "Couldn't tick the checkbox in '\(title)'" } else { "Couldn't tick the checkbox" }
         }
     }
 
@@ -655,6 +660,8 @@ public final class BannerCenter {
             "this file's frontmatter can't be edited in place (\(shape.description))"
         case let .io(message):
             message
+        case let .staleTarget(message):
+            message
         }
         return trimmed(text)
     }

Kanban/LiveStore/BoardStore.swift

@@ -1127,6 +1127,36 @@ public final class BoardStore {
         return nil
     }
 
+    // MARK: - Task checkboxes
+
+    /// Ticks or unticks a Preview task-list checkbox — **the app's one write into a card's body**
+    /// (05-card-window.md ▸ Preview), and otherwise an entirely ordinary one: the same
+    /// `performWrite` bracket, the same banner on failure, the same one-way flow back through the
+    /// watcher. "A toggle is an ordinary user edit — the standard atomic write, auto-committed and
+    /// undoable on git boards."
+    ///
+    /// `bodyOffset` is the UTF-8 byte offset the parse handed the renderer (`BodyTask
+    /// .markerOffset`) and `checked` is the state the user was looking at; both travel to
+    /// `BoardWriter.toggleTaskMarker`, which re-verifies them against the file it reads and refuses
+    /// rather than write blind. Nothing here inspects the body: the store never re-parses to
+    /// second-guess the click, because its own snapshot is exactly as stale as the render was.
+    ///
+    /// **A checkbox in a card that has gone writes nothing** — the vanished-target guard every
+    /// gesture in this file makes, ancestor-walked through `liveItem`: the card window would be
+    /// dismissing itself in the same breath, and the reload that removed the card is the authority.
+    /// The read-only lock is `performWrite`'s refusal, which is also why the controls disable in
+    /// place on the Preview side rather than failing here (02-architecture.md § the lock's scope).
+    public func toggleTaskMarker(inCard cardID: ItemID, bodyOffset: Int, checked: Bool) {
+        guard let target = Self.liveItem(cardID, in: snapshot), let card = target.cardID else { return }
+        let folder = rootURL
+            .appendingPathComponent(target.laneID.rawValue, isDirectory: true)
+            .appendingPathComponent(card.rawValue, isDirectory: true)
+
+        try? performWrite { () throws(BoardWriteError) -> Void in
+            try BoardWriter.toggleTaskMarker(inItemFolder: folder, bodyOffset: bodyOffset, checked: checked)
+        }
+    }
+
     // MARK: - Board rename
 
     /// Writes the board's own `title` — the board popover's rename field (03-board-ui.md § Board