probe(physics): ACDREAM_DUMP_TRANSIT_FAIL — self-selecting transition-phase trace for #345

Fires only on the stuck-tick predicate (>=1mm XY requested, <=0.1mm
achieved), buffering per-tick phase outcomes cheaply and flushing only
on a stuck tick: per-insert-attempt phase/state/normal/source, step-up
enter/exit verdicts, every ValidateWalkable branch with dist/waterDepth
and both SetCollisionNormal guards evaluated, and the tick's final
AdjustOffset pair. Zero cost when off (flag before any allocation — the
I1 zero-alloc gate stays green), mover id on every line, [ThreadStatic]
buffer per the referee-safety rule. Two tests: fires on a synthetic
wall-stuck tick, silent on ordinary movement.

Diagnostics only; no behavioral change. Suite 11,271 / 6 / 0.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

src/AcDream.Core/Physics/PhysicsEngine.cs

@@ -1922,6 +1922,12 @@ public sealed class PhysicsEngine
         // sentinel value for that case. No-op when the probe is off.
         PhysicsDiagnostics.BeginContactPlaneAttribution();
 
+        // #345 probe (2026-08-08): reset the per-tick transition-phase trace
+        // buffer. Scoped to exactly one resolve/tick — see
+        // PhysicsDiagnostics.BeginTransitFailTrace. No-op when the probe is
+        // off.
+        PhysicsDiagnostics.BeginTransitFailTrace();
+
         var transition = RentTransition();
         try
         {
@@ -2470,6 +2476,13 @@ public sealed class PhysicsEngine
                     Orientation: sp.CurOrientation);   // Render Residual A — the sweep failed (find_valid_position == 0)
             }
 
+            // #345 probe (2026-08-08): the self-selecting stuck-tick
+            // predicate — fires only when this tick requested a real XY
+            // move and delivered none, flushing the buffered
+            // transition-phase trace. No-op when the probe is off.
+            PhysicsDiagnostics.EmitTransitFailIfStuck(
+                movingEntityId, currentPos, targetPos, resolveResult.Position);
+
             // A6.P3 #98 capture: emit one JSON Lines record per player call,
             // with bodyBefore snapshot (taken at method entry, before any
             // engine mutation) + bodyAfter snapshot (taken now, after the