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/BoardDrops.swift

@@ -416,6 +416,12 @@ struct BoardDropContext {
     /// event time is the geometry that genuinely moves under a stationary cursor: the grid, which the
     /// autoscroll driver scrolls on purpose, and `DropSlotMath`'s own containment arithmetic.
     func retargetCards(inLane laneID: ItemID) {
+        // `DragSignposts` — begin/end with a `defer` rather than a scoped closure, so the guards
+        // below keep their early returns and this reads as instrumentation rather than a rewrite.
+        #if DEBUG
+        let span = DragSignposts.beginRetargetCards()
+        defer { DragSignposts.endRetargetCards(span) }
+        #endif
         guard session.isDraggingCards, let cursor = globalCursor() else { return }
         guard let resting = session.restingLayouts.layout(
             inLane: laneID,
@@ -732,6 +738,12 @@ struct BoardDropContext {
     /// from proposing to committed and keeps drawing the arrangement it was showing — the shadows at
     /// their landing slots, the originals lifted out — until this store's echo reload lands.
     func commitDrop() -> Bool {
+        // The synchronous half of the release, every refusing branch included (`DragSignposts`); the
+        // *pause* the user feels starts inside, at `DragSession.commit(into:)`, and outlives this.
+        #if DEBUG
+        let span = DragSignposts.beginCommitDrop()
+        defer { DragSignposts.endCommitDrop(span) }
+        #endif
         guard session.isActive, let kind = session.kind, let sourceRoot = session.sourceRoot else {
             return false
         }
@@ -944,6 +956,13 @@ struct LaneDropDelegate: DropDelegate {
     func dropEntered(info: DropInfo) { retarget(info) }
 
     func dropUpdated(info: DropInfo) -> DropProposal? {
+        // The per-event span, and the input sample (`DragSignposts`). One delegate sees any one
+        // event — single-target dispatch has no fall-through — so the cadence is not double-counted.
+        #if DEBUG
+        let span = DragSignposts.beginDropUpdated()
+        DragSignposts.sampleInput()
+        defer { DragSignposts.endDropUpdated(span) }
+        #endif
         retarget(info)
         return context.isFileSession(info) ? context.fileDropProposal() : context.dropProposal()
     }
@@ -1000,6 +1019,11 @@ struct StripDropDelegate: DropDelegate {
     func dropEntered(info: DropInfo) { retarget(info) }
 
     func dropUpdated(info: DropInfo) -> DropProposal? {
+        #if DEBUG
+        let span = DragSignposts.beginDropUpdated()
+        DragSignposts.sampleInput()
+        defer { DragSignposts.endDropUpdated(span) }
+        #endif
         retarget(info)
         return context.isFileSession(info) ? context.fileDropProposal() : context.dropProposal()
     }
@@ -1055,6 +1079,11 @@ struct TrashDropDelegate: DropDelegate {
     func dropEntered(info: DropInfo) { retarget(info) }
 
     func dropUpdated(info: DropInfo) -> DropProposal? {
+        #if DEBUG
+        let span = DragSignposts.beginDropUpdated()
+        DragSignposts.sampleInput()
+        defer { DragSignposts.endDropUpdated(span) }
+        #endif
         retarget(info)
         return context.isFileSession(info) ? context.fileDropProposal() : context.dropProposal()
     }