Build the card window shell and lifecycle

The m4 scene plumbing was already honest — one WindowGroup value per
CardWindowRef enforces one-window-per-card, and CardWindowFate's
ancestor walk answered dismissal — so this card fills the window: a
two-column shell whose body column takes all resize flex and whose
sidebar width derives once from font metrics (26 characters of average
body advance plus em gutters), the five 05-ordered section headers as
placeholders, and the card body as selectable plain text until Preview
mode lands. The fate walk now returns a CardPlacement (card + lane), so
one pass answers both liveness and the live board › lane subtitle; a
board rename lands for free through displayName. Card windows remember
their frames per card in the board record (case-folded id keys,
unchanged-writes-nothing), restoring instead of cascading; only
unremembered cards take the last-used size and cascade. Store
acquisition stays gated on liveStore — a card window never opens a
board — and the close-flush hook stands with nothing to flush until the
Edit-session card.

Claude-Session: https://claude.ai/code/session_01SR4XGjmBE16ZUYWpfFHXwY

KanbanTests/CardWindowFateTests.swift

@@ -32,8 +32,15 @@ private func makeBoard() throws -> WriterFixture {
 }
 
 private func title(_ fate: CardWindowFate) -> String? {
-    guard case let .shows(card) = fate else { return nil }
-    return card.title.value
+    guard case let .shows(placement) = fate else { return nil }
+    return placement.card.title.value
+}
+
+/// The lane the fate says the card is in — the subtitle's live half, resolved by the very same walk
+/// that decides whether the window lives at all (`CardPlacement`).
+private func laneTitle(_ fate: CardWindowFate) -> String? {
+    guard case let .shows(placement) = fate else { return nil }
+    return placement.lane.title.value
 }
 
 // MARK: - Tests
@@ -42,7 +49,7 @@ private func title(_ fate: CardWindowFate) -> String? {
 @Suite("Card window fate")
 struct CardWindowFateTests {
 
-    @Test("A live card in a live lane keeps its window")
+    @Test("A live card in a live lane keeps its window, and names the lane it is in")
     func aLiveCardShows() throws {
         let fixture = try makeBoard()
         defer { fixture.tearDown() }
@@ -50,6 +57,9 @@ struct CardWindowFateTests {
 
         let fate = CardWindowHost.cardWindowFate(cardID: Ident.card1, in: snapshot)
         #expect(title(fate) == "Fix login")
+        // The window's subtitle rides on this same resolution rather than on a second walk, so the
+        // lane it reports and the card it renders can never be one snapshot apart.
+        #expect(laneTitle(fate) == "Todo")
     }
 
     @Test("A tombstoned card dismisses its window")