fix(physics): restore nested per-cell collision retries

src/AcDream.Core/Physics/PhysicsEngine.cs

@@ -63,6 +63,18 @@ public sealed class PhysicsEngine
     /// </summary>
     public Action<string>? DiagnosticLog { get; set; }
 
+    /// <summary>
+    /// Deterministic test seam for the retail per-cell dispatcher. Production
+    /// leaves this null. Tests may observe a completed phase and substitute its
+    /// returned state to prove retry/order semantics without geometry-specific
+    /// response coupling.
+    /// </summary>
+    internal Func<
+        TransitionCellCollisionPhase,
+        uint,
+        TransitionState,
+        TransitionState>? TransitionCellCollisionTestHook { get; set; }
+
     /// <summary>
     /// True once the landblock covering <paramref name="cellOrLandblockId"/> has had its
     /// terrain + cells registered via <see cref="AddLandblock"/>. Accepts a canonical