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.

KanbanTests/TransientBoardStateTests.swift

@@ -441,6 +441,42 @@ struct TransientBoardStateTests {
         #expect(store.transient.renameEditor == RenameEditor(targetID: lane1, draftTitle: "To d"))
     }
 
+    // MARK: The cursors' sole-member default
+
+    @Test("The sole-member default is opt-out — a band that sweeps one card acquires no cursors")
+    func theSoleMemberDefaultIsOptOut() throws {
+        let fixture = try makeBoard()
+        defer { fixture.tearDown() }
+        let store = try BoardStore(rootURL: fixture.root)
+
+        // **The default, unchanged**: a one-item selection made by any ordinary route is a legitimate
+        // range origin and a legitimate place to arrow from, so `nil` takes the sole member.
+        store.transient.select([card1], in: .board)
+        #expect(store.transient.selectionAnchor == card1)
+        #expect(store.transient.selectionHead == card1)
+
+        // **The opt-out**: a rubber band names no click to range from and no item to arrow from
+        // whatever it happens to enclose, and passing `nil` cannot say so — `nil` is what *asks* for
+        // the default. `MarqueeControl.gesture` passes the flag, and this is what it buys: a band
+        // narrowed onto exactly one card leaves both cursors empty, so a ⇧-click after it acts plain
+        // and `LaneView.cardStack`'s scroll-to has no head to fire on mid-drag.
+        store.transient.select([card2], in: .board, anchor: nil, head: nil, defaultsSoleMember: false)
+        #expect(store.transient.selection.ids == [card2], "the selection itself still lands")
+        #expect(store.transient.selectionAnchor == nil)
+        #expect(store.transient.selectionHead == nil)
+
+        // The flag defaults nothing away: an explicit cursor is still taken verbatim.
+        store.transient.select([card2], in: .board, anchor: card2, head: card2, defaultsSoleMember: false)
+        #expect(store.transient.selectionAnchor == card2)
+        #expect(store.transient.selectionHead == card2)
+
+        // And the store's funnel forwards it — the marquee calls `BoardStore.select`, not the
+        // transient's directly.
+        store.select([card1], in: .board, anchor: nil, head: nil, defaultsSoleMember: false)
+        #expect(store.transient.selectionAnchor == nil)
+        #expect(store.transient.selectionHead == nil)
+    }
+
     // MARK: The last-active lane
 
     @Test("Selecting a lane or one of its cards marks it active; clearing the selection does not forget it")