perf(physics): pool collision cell snapshots

Preserve the fixed per-cell collision walk across nested registry mutations while replacing the repeated List allocation with an explicitly owned pooled snapshot. Capture cardinality once, return storage on every exit, and pin live-list mutation behavior with a focused regression test.

Co-authored-by: Codex <codex@openai.com>

tests/AcDream.Core.Tests/Physics/ShadowObjectRegistryTests.cs

@@ -16,6 +16,25 @@ public class ShadowObjectRegistryTests
     private const float OffX = 0f;
     private const float OffY = 0f;
 
+    [Fact]
+    public void ShadowEntrySnapshot_LiveListMutation_DoesNotChangeCapturedWalk()
+    {
+        var first = new ShadowEntry(1u, 0u, Vector3.Zero, Quaternion.Identity, 1f);
+        var second = new ShadowEntry(2u, 0u, Vector3.One, Quaternion.Identity, 1f);
+        var replacement = new ShadowEntry(3u, 0u, Vector3.UnitX, Quaternion.Identity, 1f);
+        var liveEntries = new List<ShadowEntry> { first, second };
+
+        using var snapshot = ShadowEntrySnapshot.Capture(liveEntries);
+
+        liveEntries.RemoveAt(0);
+        liveEntries.Add(replacement);
+        liveEntries.Add(first);
+
+        Assert.Equal(
+            new uint[] { first.EntityId, second.EntityId },
+            snapshot.Entries.ToArray().Select(entry => entry.EntityId));
+    }
+
     // -----------------------------------------------------------------------
     // Register / TotalRegistered
     // -----------------------------------------------------------------------