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/HistoryProviderTests.swift

@@ -703,7 +703,7 @@ struct UndoCommandSurfaceTests {
             defer: true
         )
         let controller = HostedWindowController()
-        controller.boardUndoManager = { manager }
+        controller.windowUndoManager = { manager }
         controller.attach(to: window)
         return (window, controller)
     }
@@ -792,8 +792,11 @@ struct UndoCommandSurfaceTests {
         #expect(undoRow.title == "Undo")
     }
 
-    @Test("Every window over one board answers with that board's one stack")
-    func cardWindowsShareTheBoardsStack() throws {
+    @Test("A board window answers with the board's stack; a card window answers with its own")
+    func eachWindowAnswersWithItsOwnStack() throws {
+        // Realigned 2026-07-31 with the two-level model (13-native-undo.md ▸ Rules): this pinned the
+        // shared-stack wiring, which is exactly what the session-coarsening re-ruling replaced —
+        // "a card window owns its own stack ... and `window.undoManager` answers with it".
         let fixture = try makeBoard()
         defer { fixture.tearDown() }
         let (model, tearDown) = try makeModel()
@@ -801,10 +804,12 @@ struct UndoCommandSurfaceTests {
 
         let ref = try openBoard(model, at: fixture.root)
         let session = try #require(model.session(for: ref))
-        // The board window and one of its card windows, wired the way their hosts wire them.
+        // The board window and one of its card windows, wired the way their hosts wire them
+        // (`BoardWindowHost.configureWindow`, `CardWindowHost.configureWindow`).
+        let card = CardWindowUndo()
         let (boardWindow, boardController) = hostedWindow(session.undoManager)
         defer { boardController.detach() }
-        let (cardWindow, cardController) = hostedWindow(session.undoManager)
+        let (cardWindow, cardController) = hostedWindow(card.manager)
         defer { cardController.detach() }
         let boardRow = menuItem("undo:")
         let cardRow = menuItem("undo:")
@@ -812,9 +817,17 @@ struct UndoCommandSurfaceTests {
         session.history?.register(StepLog().step("Move 3 Cards"))
 
         #expect(boardWindow.validateMenuItem(boardRow))
-        #expect(cardWindow.validateMenuItem(cardRow), "one stack per board, not per window")
         #expect(boardRow.title == "Undo Move 3 Cards")
-        #expect(cardRow.title == boardRow.title)
+
+        // **No fall-through** (06-history-undo.md ▸ Undo routing): the card window's own stack is
+        // empty, so its row is disabled and ⌘Z beeps — it never reaches the board's step.
+        #expect(cardWindow.validateMenuItem(cardRow) == false)
+        #expect(cardRow.title == "Undo")
+
+        card.stack.register(StepLog().step("Comment"))
+        #expect(cardWindow.validateMenuItem(cardRow))
+        #expect(cardRow.title == "Undo Comment")
+        #expect(boardRow.title == "Undo Move 3 Cards", "and the board's row is untouched by it")
     }
 
     // MARK: The toolbar twins