Give card windows their own undo stacks and coarsen the close

Phase B of the two-level undo card: every card-window gesture — comment
post/delete/edit, body Edit sessions, style and details changes —
registers fine-grained on the window's own stack (window.undoManager
answers with it; board ⌘Z never sees mid-session card steps; an empty
window stack beeps, never falls through). Window close folds the stack
into one coarse values-based board step ("Edit card 'X'") — per-target
per-field later-wins merge, so foreign mid-session writes stay out by
construction, a no-net-change session registers nothing, and any stale
component skips the whole step. The comments/.trash purge defers with
the coarse step via a step-retirement seam on the providers: it runs
when the step leaves the board stack or the board session ends; the git
provider retires dropped steps on register, which keeps Pro's
purge-at-close-flush structural with no tier check. Interim on git
boards: gestures still auto-commit per debounce until phase C's
close-flush commit.

2432 tests in 418 suites green.

Claude-Session: https://claude.ai/code/session_01CqjXB7ASoWtbyoGod68k97

Kanban/UI/Card/CardWindowView.swift

@@ -91,6 +91,11 @@ struct CardWindowView: View {
     let comments: CardComments
     /// This window's thumbnail memory, held by the host so it outlives a snapshot.
     let thumbnails: AttachmentThumbnailCache
+    /// **This window's undo stack** (13-native-undo.md ▸ Rules ▸ two levels). It arrives for exactly
+    /// one consumer — the Style section, the one sidebar anchor that *writes* — because a gesture
+    /// issued in this window registers on this window's stack. It lives on the window's session so
+    /// the close can fold it, which is why it arrives here rather than being made here.
+    let undo: CardWindowUndo
     /// **This card's commit trail** (05 ▸ History), or `nil` on every board with no app-managed git —
     /// the free tier, mode none, and repo-nested boards. The `nil` *is* the section's absence rule;
     /// see `historySlot`.
@@ -314,7 +319,7 @@ struct CardWindowView: View {
             VStack(alignment: .leading, spacing: bodyPointSize * 1.25) {
                 CardAttachmentsSection(attachments: attachments, thumbnails: thumbnails)
 
-                CardStyleSection(store: store, recents: recents, cardID: card.id)
+                CardStyleSection(store: store, recents: recents, cardID: card.id, undo: undo)
 
                 // The snapshot's own document, not a re-read: the loader parsed this file, unknown
                 // keys and their order included, and `Card` has carried it since (`BoardModel`).