Selected-ness rides down as a compared parameter — a marquee crossing repaints its faces, not the board

A selection change re-ran every CardFaceView on the board (180 bodies ≈ 85 ms on
the 6×30 fixture, 515 ≈ 233 ms on a real 515-card board, debug): the face's body
read store.selection in three places — isSelected, the drag replica's count, and
the context menu's styleTarget — and Observation invalidates every reader of the
property, past the equatable gate entirely. The band overlay stayed cheap, which
is why the marquee tracked the cursor while the highlight lagged ~0.4 s behind.

Now LaneView and TrashLaneView hoist one selection read per body and hand each
face isSelected/selectedCount as compared parameters; StyleMenuItems takes its
target as a deferred closure; TrashLaneRowView gains the same treatment plus the
Equatable gate it never needed before. Select-one-card: 180 bodies → 1. A
growing band costs the selection's own running size; the real board's crossing
fell 233 → 112 ms — the remainder is lane bodies re-measuring their masonry, a
separate lane-level finding recorded in RENDER-INSTRUMENTATION.md.

Also: select() gains defaultsSoleMember — the marquee's explicit nils never
avoided the sole-member default, so a one-card band acquired a selectionHead and
could scroll the lane out from under its own drag.

MarqueeRenderCostTests pins the shape: redundant samples cost zero bodies, a
growing band pays per crossing, and selectionStillRepaints holds a ≤8 budget.

Kanban/UI/StyleEditor.swift

@@ -656,15 +656,21 @@ private struct StyleWellGrid: View {
 /// One view for both menus because the entries are identical on a card and on a lane: only the
 /// *target* differs, and that is the caller's to compute (the clicked item, or the selection it
 /// belongs to).
+///
+/// **The target arrives as a closure, deliberately.** `.contextMenu`'s builder is non-escaping, so a
+/// caller's menu is assembled while its own body runs — and a target computed eagerly is a
+/// `store.selection` read at body time, which under Observation subscribes that body to every
+/// selection change on the board. Deferring it means a card face's context menu costs its face
+/// nothing until a row actually acts (`CardFaceView.isSelected` has the measurement).
 struct StyleMenuItems: View {
 
     let store: BoardStore
     let recents: StyleRecents
-    let target: StyleTarget
+    let target: () -> StyleTarget
 
     var body: some View {
         Button("Style…") {
-            store.transient.beginStyleEditor(for: target)
+            store.transient.beginStyleEditor(for: target())
         }
         .disabled(!store.acceptsBoardMutations)
 
@@ -683,11 +689,15 @@ struct StyleMenuItems: View {
 ///
 /// **Absent until it has something to offer.** A brand-new install has no recents, and an empty
 /// picker in a context menu is a row that looks broken.
+///
+/// The target is deferred for `StyleMenuItems`' reason, and this row is where the deferral has to
+/// hold: the `Binding` below is read when the menu shows a checkmark and written when a dot is
+/// picked, both of them the picker's own doing rather than the enclosing card face's body.
 struct QuickStyleRow: View {
 
     let store: BoardStore
     let recents: StyleRecents
-    let target: StyleTarget
+    let target: () -> StyleTarget
 
     /// A sentinel for "the current value is not one of these", so a mixed batch — or a background
     /// that has aged out of the recents — leaves the row unchecked rather than checking the wrong
@@ -717,13 +727,13 @@ struct QuickStyleRow: View {
     private var selection: Binding<Choice> {
         Binding(
             get: {
-                let state = StyleFieldState.resolve(store.styleSubjects(of: target).map(\.background))
+                let state = StyleFieldState.resolve(store.styleSubjects(of: target()).map(\.background))
                 guard case let .uniform(value) = state, recents.backgrounds.contains(value) else { return .other }
                 return .value(value)
             },
             set: { picked in
                 guard case let .value(name) = picked else { return }
-                StyleCommand.apply(background: .set(name), to: target, in: store, recents: recents)
+                StyleCommand.apply(background: .set(name), to: target(), in: store, recents: recents)
             }
         )
     }