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

KanbanTests/CommentAnnouncementTests.swift

@@ -276,7 +276,9 @@ struct CommentPaneAnnouncementTests {
         comments.isEditable = true
         comments.cardFolder = fixture.url("\(Ident.lane1)/\(Ident.card1)")
         comments.cardTitle = "Fix login"
-        CardWindowHost.configureComments(comments, store: store, cardID: ItemID(rawValue: Ident.card1))
+        CardWindowHost.configureComments(
+            comments, store: store, cardID: ItemID(rawValue: Ident.card1), on: CardWindowUndo()
+        )
         let spoken = Spoken(store: store)
         comments.announce = { spoken.record($0) }
         return (comments, spoken)