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/LiveStore/BoardStore.swift

@@ -4545,8 +4545,19 @@ public final class BoardStore: HealHost {
     /// for "the lane that most recently held selection or a creation", and a *card* selection is
     /// its lane holding selection just as much as the lane's own header click is — so both are
     /// noted here, and creation notes itself in `beginPlaceholder`.
-    public func select(_ ids: Set<ItemID>, in container: ItemContainer, anchor: ItemID? = nil, head: ItemID? = nil) {
-        transient.select(ids, in: container, anchor: anchor, head: head)
+    ///
+    /// - Parameter defaultsSoleMember: forwarded verbatim — see `TransientBoardState.select`, whose
+    ///   sole-member default the rubber band opts out of.
+    public func select(
+        _ ids: Set<ItemID>,
+        in container: ItemContainer,
+        anchor: ItemID? = nil,
+        head: ItemID? = nil,
+        defaultsSoleMember: Bool = true
+    ) {
+        transient.select(
+            ids, in: container, anchor: anchor, head: head, defaultsSoleMember: defaultsSoleMember
+        )
         transient.noteActiveLane(Self.lane(holding: ids, in: snapshot))
     }