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/BoardWindowHost.swift

@@ -264,6 +264,13 @@ struct BoardWindowHost: View {
             }
         }
 
+        // This window's answer to "what does ⌘Z act on" (13-native-undo.md ▸ Rules; 06 ▸ Undo
+        // routing) — the *session's* stack, read afresh on every ask so a torn-down board answers
+        // 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 }
+
         // 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
         // card windows share that machinery and have no board to describe. It goes in after the