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/MasonryLayout.swift

@@ -210,6 +210,10 @@ struct MasonryLayout: Layout {
     // change, which is the only way a card's measured height can change at a fixed column width
     // (its content changed → its view tree was rebuilt → the layout's content is new). Clearing
     // there means the cache never outlives the content it measured.
+    //
+    // Both halves are counted in DEBUG builds (`BoardRenderMetrics.masonryMeasurements` /
+    // `masonryCacheHits`), which is what makes "the cache absorbs SwiftUI's repeat probes" a number
+    // a test can read rather than a claim — see RENDER-INSTRUMENTATION.md.
     struct Cache {
         var columnWidth: CGFloat = .nan
         var heights: [CGFloat] = []
@@ -229,8 +233,14 @@ struct MasonryLayout: Layout {
             cache.heights = [CGFloat](repeating: .nan, count: subviews.count)
         }
         if !cache.heights[index].isNaN {
+            #if DEBUG
+            BoardRenderMetrics.masonryCacheHits += 1
+            #endif
             return cache.heights[index]
         }
+        #if DEBUG
+        BoardRenderMetrics.masonryMeasurements += 1
+        #endif
         let height = subviews[index].sizeThatFits(ProposedViewSize(width: column, height: nil)).height
         cache.heights[index] = height
         return height
@@ -248,6 +258,9 @@ struct MasonryLayout: Layout {
     }
 
     func sizeThatFits(proposal: ProposedViewSize, subviews: Subviews, cache: inout Cache) -> CGSize {
+        #if DEBUG
+        BoardRenderMetrics.masonrySizeThatFitsCalls += 1
+        #endif
         let width = proposal.width ?? 0
         let placement = placement(width: width, origin: .zero)
         let heights = measuredHeights(of: subviews, at: placement.columnWidth, cache: &cache)
@@ -255,6 +268,9 @@ struct MasonryLayout: Layout {
     }
 
     func placeSubviews(in bounds: CGRect, proposal: ProposedViewSize, subviews: Subviews, cache: inout Cache) {
+        #if DEBUG
+        BoardRenderMetrics.masonryPlaceCalls += 1
+        #endif
         let placement = placement(width: bounds.width, origin: bounds.origin)
         let heights = measuredHeights(of: subviews, at: placement.columnWidth, cache: &cache)
         for (index, frame) in placement.frames(heights: heights).enumerated() {