Build Edit mode with debounced, byte-honest saves

The editing surface: the same hosted TextKit-1 text view gains an
editable branch with a per-keystroke line-scanner highlighter — chosen
over a parser re-parse because a mid-typing buffer is usually invalid
Markdown and 05 wants the delimiters themselves dimmed; apply only sets
attributes, so presentation-never-transforms is structural. Saves ride
a ~700ms injectable debounce through BoardWriter.writeBody —
toggleTaskMarker's idiom widened to the body span, frontmatter bytes
untouched, refusing to write when disk already holds that body, which
enforces all three gates (untouched, reverted, echo) at the layer that
owns the bytes with one isDirty predicate above it. Mode grammar lands
whole: ⌘E toggles with a checkmark, Return in Preview enters, Escape
returns, and every flip flushes first; window close flushes through
the existing retry/save-copy/discard modal, and the dismissal flush
deliberately reaches a tombstoned card. Dirty-buffer-wins: disk always
follows the snapshot, the buffer only when clean, both surfaces render
the buffer. Undo is the editor's own session-scoped NSUndoManager;
endEditSession names the pro-m1 one-commit-per-session boundary.

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

README.md

@@ -34,6 +34,8 @@ Lanework is in early development. This list tracks what has actually shipped and
 
 - **The card window** — ⌘↩ or a double-click opens a card in its own window: two full-height, independently scrolling columns — a wide body column and a narrow attributes sidebar whose fixed width is derived from font metrics, so the window's resize flex all goes to the body. The title bar carries the card's title live and subtitles it "⟨board⟩ › ⟨lane⟩", following the card as it moves between lanes and re-reading the board's name as it's renamed. The body column shows the title, a quiet created/modified/by line built from whichever frontmatter keys exist, and the body itself; the sidebar's sections are stacked headers awaiting their content. Reopening a live card focuses the window it already has, and the window closes itself the moment its card stops being live — deleted, tombstoned, buried under a tombstoned lane, or moved to another board.
 
+- **The card body — Preview and Edit** — the body is read as a fully rendered Markdown preview and written as raw Markdown, never a WYSIWYG halfway house. Preview lays out headings, emphasis, code, quotes, lists, GFM tables, thematic breaks and images resolved against the card's own folder; HTML shows verbatim as code, remote images never load (Preview does no networking), links open in the browser or the file's default app, and task-list checkboxes are live — clicking one flips exactly that character in the file and touches no other byte. ⌘E toggles View ▸ Edit Body, Return in Preview enters it, Escape leaves it, and a card whose body is empty opens straight into the editor with the cursor ready. Edit is a monospaced editor with lightweight syntax highlighting — headings emphasized, bold and italic styled, code tinted, link targets and structural markers dimmed — that is presentation only: the text stays the raw Markdown character for character, smart quotes and dashes off. It saves ~700 ms after you stop typing, and flushes the moment you leave Edit or close the window, so neither the preview nor the disk ever lags what you typed. ⌘Z is the editor's own undo, scoped to the session; ⌘F is find-in-text over whichever surface is showing. Three write rules keep the file honest: a body nobody touched is never re-serialized (byte-identical on disk, modification date included), an edit typed and then undone is not written, and the app's own save echoing back through the watcher is not written again. If the file changes underneath you while the buffer has unsaved keystrokes, the buffer wins — the board, the preview and every other window take the new version while your text stays exactly where it is, and your save then lands over theirs. A close that cannot save stops and asks: try again, save a copy elsewhere, or discard.
+
 - **The board popover** — a quiet chevron beside the window title (File ▸ Board Info, ⌘I, which toggles it) opens the board's one configuration surface. Renaming edits the board's frontmatter `title` and nothing else — the folder is never renamed, so the app's display name and the Finder document name are free to diverge — and clearing the field removes the key entirely, dropping the window title back to the folder name rather than to "Untitled"; the edit commits on Return and on click-away, Escape abandons it, and an unchanged title writes nothing at all. Below it sits the same style editor every other anchor uses, aimed permanently at the board, and below that a Git section that says plainly that this board has no repository yet. The read-only lock disables the surface without closing it.
 
 ## Development