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/UI/StyleEditor.swift

@@ -30,14 +30,18 @@ import SwiftUI
 /// colour to remember; only `.set` reaches `StyleRecents.record`.
 @MainActor
 enum StyleCommand {
+    /// - Parameter undo: the issuing window's own stack, for the one anchor that has one — the card
+    ///   window's sidebar (13-native-undo.md ▸ Rules ▸ two levels). `nil`, which every board-side
+    ///   anchor passes, is the board's stack.
     static func apply(
         background: StyleChange = .keep,
         icon: StyleChange = .keep,
         to target: StyleTarget,
         in store: BoardStore,
-        recents: StyleRecents
+        recents: StyleRecents,
+        on undo: CardWindowUndo? = nil
     ) {
-        store.applyStyle(to: target, background: background, icon: icon)
+        store.applyStyle(to: target, background: background, icon: icon, on: undo)
         if case let .set(value) = background {
             recents.record(value)
         }
@@ -239,6 +243,13 @@ struct StyleEditorView: View {
     /// size and a default argument cannot read one.
     var layout: StyleEditorLayout?
 
+    /// **Which stack this anchor's writes register on** — the card window's own when the editor is
+    /// mounted in one (`CardStyleSection`), and `nil`, the board's, everywhere else
+    /// (13-native-undo.md ▸ Rules ▸ two levels). It sits beside `layout` and arrives the same way —
+    /// the anchor telling the shared component about itself — but unlike `layout` it is not geometry:
+    /// a colour chosen in a card window is one of that window's session gestures.
+    var undo: CardWindowUndo?
+
     /// The live body metric, read here rather than passed in — `CardStyleSection`'s pattern, so
     /// every anchor derives its geometry the same way (10-accessibility.md's full-relative-scaling
     /// rule).
@@ -293,7 +304,7 @@ struct StyleEditorView: View {
                 columns: layout.backgroundColumns,
                 layout: layout,
                 apply: { change in
-                    StyleCommand.apply(background: change, to: target, in: store, recents: recents)
+                    StyleCommand.apply(background: change, to: target, in: store, recents: recents, on: undo)
                 }
             )
         }
@@ -346,7 +357,7 @@ struct StyleEditorView: View {
             columns: layout.symbolColumns,
             layout: layout,
             apply: { change in
-                StyleCommand.apply(icon: change, to: target, in: store, recents: recents)
+                StyleCommand.apply(icon: change, to: target, in: store, recents: recents, on: undo)
             }
         )
         if let maximumHeight = layout.symbolGridMaximumHeight {