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/Card/CardSidebarSections.swift

@@ -32,6 +32,11 @@ struct CardStyleSection: View {
     let store: BoardStore
     let recents: StyleRecents
     let cardID: ItemID
+    /// **This window's undo stack** (13-native-undo.md ▸ Rules ▸ two levels): a colour or symbol
+    /// chosen here is a gesture *issued in this window*, so its step joins the window's session and
+    /// reaches board history only inside the coarse close step. The shared editor takes it as an
+    /// anchor's parameter, exactly as it takes the layout.
+    let undo: CardWindowUndo
 
     /// The live body metric, read here rather than passed in — `CardAttachmentsSection`'s pattern,
     /// so every section in this sidebar derives its geometry the same way.
@@ -58,7 +63,8 @@ struct CardStyleSection: View {
                 layout: .sidebar(
                     contentWidth: CardWindowMetrics.sidebarContentWidth(bodyPointSize: pointSize),
                     bodyPointSize: pointSize
-                )
+                ),
+                undo: undo
             )
         }
         .frame(maxWidth: .infinity, alignment: .leading)