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

Kanban/LiveStore/BannerCenter.swift

@@ -646,6 +646,12 @@ public final class BannerCenter {
             // 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" }
+        case let .editBody(title):
+            // **Save**, because that is the word for what just failed: the Edit→Preview flip is the
+            // effective Save button (05-card-window.md ▸ Edit), and the debounced tick is the same
+            // act happening on its own. The keystrokes are still in the buffer — the banner says the
+            // app could not put them on disk, not that they are gone.
+            if let title { "Couldn't save '\(title)'" } else { "Couldn't save the card" }
         }
     }