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/Board/SelectionClicks.swift

@@ -122,10 +122,16 @@ struct MarqueeControl {
                     // Neither cursor: a band names no click to range from and no item to arrow from,
                     // so a ⇧-click after one acts plain and an arrow re-derives a position from the
                     // set's last member (`TransientBoardState.selectionAnchor`, `selectionHead`).
-                    // Both are spelled out rather than defaulted, because `select`'s sole-member
-                    // default would otherwise pick one up the moment a band happened to sweep
-                    // exactly one card.
-                    store.select(ids, in: session.container, anchor: nil, head: nil)
+                    //
+                    // **`defaultsSoleMember: false` is what says that, and the explicit `nil`s never
+                    // did.** `nil` *is* the default, so a band that swept exactly one card used to
+                    // pick up both cursors anyway — harmless for the anchor, not for the head:
+                    // `LaneView.cardStack` watches `selectionHead` and scrolls the lane to it, which
+                    // means a one-card band could scroll the board out from under the drag that was
+                    // drawing it. The flag is the only way to spell "no gesture named these".
+                    store.select(
+                        ids, in: session.container, anchor: nil, head: nil, defaultsSoleMember: false
+                    )
                 }
             }
             .onEnded { _ in session.end() }