perf(physics): reuse reset-complete transition scratch

Mirror retail's ten-deep LIFO transition lifetime, retain all query scratch with complete reset contracts, and remove Tier-0 enum boxing without changing collision decisions. Fresh and retained engines are bit-identical across the expanded oracle, while measured transition profiles now allocate 0 bytes per resolve.

src/AcDream.Core/Physics/CellTransit.cs

@@ -705,26 +705,38 @@ public static class CellTransit
         out IReadOnlyCollection<uint> cellSet,
         Vector3? carriedBlockOrigin = null)
     {
+        var candidates = new CellArray();
         var containing = BuildCellSetAndPickContaining(
             cache, worldSpheres, numSpheres, currentCellId,
-            carriedBlockOrigin, out var candidates);
+            carriedBlockOrigin, candidates);
         cellSet = candidates;
         return containing;
     }
 
+    internal static uint FindCellSet(
+        PhysicsDataCache cache,
+        IReadOnlyList<Sphere> worldSpheres,
+        int numSpheres,
+        uint currentCellId,
+        CellArray candidates,
+        Vector3? carriedBlockOrigin = null)
+        => BuildCellSetAndPickContaining(
+            cache, worldSpheres, numSpheres, currentCellId,
+            carriedBlockOrigin, candidates);
+
     private static uint BuildCellSetAndPickContaining(
         PhysicsDataCache cache,
         IReadOnlyList<Sphere> worldSpheres,
         int numSpheres,
         uint currentCellId,
         Vector3? carriedBlockOrigin,
-        out CellArray candidates)
+        CellArray candidates)
     {
         // Ordered, deduped candidate array — retail CELLARRAY (add_cell @701036).
         // The ORDER is load-bearing: the current cell is added at index 0 and the
         // pick iterates in order with interior-wins-break, so the current cell wins
         // a boundary straddle and the membership does not ping-pong (the R1 flap).
-        candidates = new CellArray();
+        candidates.Clear();
         int sphereCount = EffectiveSphereCount(worldSpheres, numSpheres);
         if (sphereCount == 0) return currentCellId;