Drag instrumentation — render counters, hot-path signposts, and the invariants that prove the gates

BoardRenderMetrics (DEBUG-only, the pathfinder's counter bag plus a
strip-body discriminator that tells a failed gate from a direct
Observation invalidation) counted at BoardView/LaneView/CardFaceView/
TrashLaneView bodies and MasonryLayout's callbacks. DragSignposts wraps
dropUpdated, retargetCards, commitDrop, and the commit-to-covering-
snapshot release pause; input latency reports honestly against
NSApp.currentEvent's mach base or labels itself base=none — no event
timestamp rides the drop path. BoardRenderPerformanceTests hosts the
real BoardView off-screen: a value-equal reload runs zero lane and zero
card bodies, a one-card edit repaints one card of 180. The lane-body
budget is <= laneCount with the headerInk chain documented and a
two-way tripwire that fails when the fix lands. Methodology in
RENDER-INSTRUMENTATION.md. Release build proves it all compiles out.

Drag-perf card a450ad09.

Claude-Session: https://claude.ai/code/session_01CqjXB7ASoWtbyoGod68k97

Kanban/UI/Board/TrashLaneView.swift

@@ -126,6 +126,11 @@ struct TrashLaneView: View {
     private var cardSpacing: CGFloat { BoardMetrics.cardSpacing(bodyPointSize: pointSize) }
 
     var body: some View {
+        // The strip's third body level, observed (`BoardRenderMetrics`) — DEBUG only, and a
+        // `let _` because `body` is a `@ViewBuilder` and a bare `Void` call is not a view.
+        #if DEBUG
+        let _ = BoardRenderMetrics.countTrashLaneBody()
+        #endif
         VStack(alignment: .leading, spacing: 0) {
             header
             cards