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/History/HistoryStaleness.swift

@@ -36,6 +36,31 @@ public enum ExpectedField: Sendable, Equatable {
     /// The body span, **byte for byte** — the Edit session's step, and the one inverse in the app
     /// whose fidelity is not field-level (13: "body steps compare bytes").
     case body(String)
+
+    /// Which field this is, ignoring the value it carries — the key a fold merges on
+    /// (`CardWindowUndo`): two writes to `background` inside one card-window session are one field
+    /// with a first and a last value, while a write to `background` and one to `icon` are two.
+    var kind: Kind {
+        switch self {
+        case .title: .title
+        case .order: .order
+        case .width: .width
+        case .background: .background
+        case .icon: .icon
+        case .body: .body
+        }
+    }
+
+    /// The field names, as a comparable value — deliberately not the `String` keys, which are
+    /// `FrontmatterKeys`' business and would tie a fold to the file format.
+    enum Kind: Hashable, Sendable {
+        case title
+        case order
+        case width
+        case background
+        case icon
+        case body
+    }
 }
 
 // MARK: - HistoryExpectation