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/Git/GitHistoryProvider.swift

@@ -247,12 +247,22 @@ public final class GitHistoryProvider: HistoryProviding {
 
     // MARK: - HistoryProviding
 
-    /// **Deliberately nothing.** On a git board an undo step is a commit, and the Writer boundary's
-    /// inverse operations are the *free* tier's substrate (13-native-undo.md). `BoardStore` registers
-    /// against whatever provider the session bound, and this one has a repository to read instead —
-    /// so the registrations arrive and are dropped, which is exactly what "the commit trail itself is
-    /// the substrate" (14 ▸ C1) means in code.
-    public func register(_ step: HistoryStep) {}
+    /// **Deliberately nothing — except the one thing a dropped step is owed.** On a git board an undo
+    /// step is a commit, and the Writer boundary's inverse operations are the *free* tier's substrate
+    /// (13-native-undo.md). `BoardStore` registers against whatever provider the session bound, and
+    /// this one has a repository to read instead — so the registrations arrive and are dropped, which
+    /// is exactly what "the commit trail itself is the substrate" (14 ▸ C1) means in code.
+    ///
+    /// Dropping a step means **retiring** it (`HistoryStep.Retirement`), and that is what keeps the
+    /// tier split in 13's purge rule structural rather than conditional: "on Pro the substrate is
+    /// history: the close commit nets delete-plus-purge to a removal, revert restores it, so purge
+    /// rides the close flush there as before" (13 ▸ Interaction with the trash). A card window's close
+    /// step registered here is retired on arrival, so its deferred `comments/.trash/` purge runs
+    /// immediately — at the close flush, exactly where it ran before this milestone — with no call
+    /// site anywhere asking which substrate it is talking to.
+    public func register(_ step: HistoryStep) {
+        step.retirement?.run()
+    }
 
     public var canUndo: Bool {
         guard !isCrossing, isHeld?() != true else { return false }