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/TransitionScratchArena.cs

@@ -0,0 +1,68 @@
+using System;
+using System.Threading;
+
+namespace AcDream.Core.Physics;
+
+/// <summary>
+/// Stack-disciplined reusable transition scratch owned by one
+/// <see cref="PhysicsEngine"/>.
+///
+/// Retail <c>CTransition::makeTransition</c> (0x0050B150) leases one of ten
+/// pre-existing records by nesting depth, calls <c>CTransition::init</c>, and
+/// releases it through <c>CTransition::cleanupTransition</c> (0x00509DC0).
+/// This arena keeps the same ten-deep, LIFO, same-thread contract while
+/// lazily constructing records that are actually used.
+/// </summary>
+internal sealed class TransitionScratchArena
+{
+    internal const int Capacity = 10;
+
+    private readonly Transition?[] _records = new Transition?[Capacity];
+    private int _depth;
+    private int _ownerThreadId;
+
+    internal int ActiveDepth => _depth;
+
+    internal Transition Rent()
+    {
+        int threadId = Environment.CurrentManagedThreadId;
+        if (_depth != 0 && _ownerThreadId != threadId)
+        {
+            throw new InvalidOperationException(
+                "Physics transition scratch cannot be shared across threads.");
+        }
+
+        if (_depth >= Capacity)
+        {
+            throw new InvalidOperationException(
+                $"Physics transition nesting exceeds retail's {Capacity}-record scratch arena.");
+        }
+
+        if (_depth == 0)
+            _ownerThreadId = threadId;
+
+        Transition transition = _records[_depth] ??= new Transition();
+        _depth++;
+        transition.ResetForReuse();
+        return transition;
+    }
+
+    internal void Return(Transition transition)
+    {
+        ArgumentNullException.ThrowIfNull(transition);
+
+        int threadId = Environment.CurrentManagedThreadId;
+        int index = _depth - 1;
+        if (index < 0
+            || _ownerThreadId != threadId
+            || !ReferenceEquals(_records[index], transition))
+        {
+            throw new InvalidOperationException(
+                "Physics transition scratch must be returned in LIFO order on its owning thread.");
+        }
+
+        _depth = index;
+        if (_depth == 0)
+            _ownerThreadId = 0;
+    }
+}