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/DragSession.swift

@@ -577,6 +577,10 @@ final class DragSession {
         source: BoardStore,
         mixesKinds: Bool
     ) {
+        // A new drag's first `sinceLastMs` must not be the gap since the previous drag's last sample.
+        #if DEBUG
+        DragSignposts.resetInputSampling()
+        #endif
         endHold()
         restingLayouts.clear()
         self.kind = kind
@@ -664,6 +668,12 @@ final class DragSession {
     /// that guard reads.
     func commit(into store: BoardStore) {
         guard isActive else { return }
+        // **The release pause starts here** (`DragSignposts`): this is the one path that arms a hold,
+        // so every begin has an end — a refused release opens nothing. Closed at `handOff`,
+        // `expire`, or `endHold`, each with its own outcome.
+        #if DEBUG
+        DragSignposts.beginReleasePause()
+        #endif
         sourceStore?.transient.dragMembers = .empty
         watchdog?.cancel()
         watchdog = nil
@@ -688,6 +698,9 @@ final class DragSession {
     /// already been handed off — or replaced by a second drag's — is not this one's to end.
     func expire(_ hold: CommittedHold) {
         guard self.hold == hold else { return }
+        #if DEBUG
+        DragSignposts.endReleasePause(outcome: "timeout")
+        #endif
         withAnimation(Motion.dragReflow(reduced: Motion.prefersReducedMotion)) { end() }
     }
 
@@ -697,10 +710,20 @@ final class DragSession {
     /// the board it belongs to — a reload on some other board says nothing about this one.
     func handOff(root: BoardRootKey, generation: Int) {
         guard let hold, hold.isRetired(byRoot: root, generation: generation) else { return }
+        // The span the release-pause signpost exists for: commit → the covering snapshot.
+        #if DEBUG
+        DragSignposts.endReleasePause(outcome: "echo")
+        #endif
         end()
     }
 
     private func endHold() {
+        // A no-op for the two paths above, which have already closed their interval with a more
+        // specific outcome; the backstop for every other teardown (`end()` from the watchdog, a
+        // cancel, a second drag beginning).
+        #if DEBUG
+        DragSignposts.endReleasePause(outcome: "cleared")
+        #endif
         hold = nil
         holdTimeoutTask?.cancel()
         holdTimeoutTask = nil