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/App/BoardWindowHost.swift

@@ -276,7 +276,7 @@ struct BoardWindowHost: View {
         // nothing rather than a stack with no board behind it. The Edit menu's Undo/Redo rows and
         // the toolbar's pair are nil-target `undo:`/`redo:`, so this one line is what lights them
         // up: `NSWindow` validates and crosses them against exactly this manager.
-        windowController.boardUndoManager = { appModel.session(for: ref)?.undoManager }
+        windowController.windowUndoManager = { appModel.session(for: ref)?.undoManager }
 
         // The window-title widget (03-board-ui.md § Board popover) — **board windows only**, which
         // is why it is installed here rather than in `WindowAccessor`: welcome, the bootstrap and