Build the HistoryProviding seam and the per-board native undo stack

The provider seam 12 promised: HistoryProviding speaks 13's vocabulary
— register a HistoryStep (bare 06 phrase plus undo/redo closures
returning applied or skipped), canUndo/canRedo, action names, clear —
and no UndoManager type appears anywhere in it, proven by a fake that
satisfies the seam with counters. The base provider wraps a private
UndoManager with groupsByEvent off so coalescing stays the Writer call
site's decision; undo re-registers the reversed step from inside the
undo, which makes a stale-skipped step vanish for free and the
crossing loop fall through to the next. BoardUndoManager adapts the
protocol to the responder chain — a stackless UndoManager subclass
answering from the provider — so Pro's git provider inherits menu
enablement, dynamic titles, and the nil-target toolbar pair by binding
the protocol. One stack per board session, born in beginSession,
cleared in the close flush; every window over the board answers it
through windowWillReturnUndoManager. Headless probes shaped the
routing: a real NSTextView's own manager wins natively, but a field
editor's does not — BoardUndoRouting answers the per-window text
manager while any NSText is first responder, so a search-field typo
never crosses a board step.

Claude-Session: https://claude.ai/code/session_01SR4XGjmBE16ZUYWpfFHXwY

Kanban/App/CardWindowHost.swift

@@ -523,6 +523,15 @@ struct CardWindowHost: View {
             CardToolbar.controller(body: bodyPresentation, rawSource: rawSource, attachments: attachments)
         )
 
+        // **The board's stack, not one of this window's own** (13-native-undo.md ▸ Rules: "not
+        // per-window: every window over a board (board window, its card windows) shares the store
+        // and shares the stack"). Same closure shape as the board window's, and deliberately the
+        // same object: ⌘Z with a card window in front crosses the board step the user last made,
+        // wherever they made it. The card's *text* surfaces are untouched by this — the body editor
+        // and the raw-source editor each vend their own manager to the responder chain, which is
+        // what keeps typing undo out of the board's stack (06-history-undo.md ▸ Undo routing).
+        windowController.boardUndoManager = { appModel.session(for: ref.board)?.undoManager }
+
         windowController.onAttach = { window in
             if let recordID,
                let saved = appModel.boardRegistry.cardWindowFrame(id: recordID, cardID: ref.cardIdentity) {