feat(streaming): PhysicsDatBundle on LoadedLandblock (datLock fix scaffold)

Carries the parsed dat objects ApplyLoadedTerrainLocked needs so the worker
can pre-read them and the apply can run lock-free. Optional field (default
null) keeps existing LoadedLandblock construction back-compatible.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/AcDream.Core/World/LoadedLandblock.cs

@@ -5,4 +5,5 @@ namespace AcDream.Core.World;
 public sealed record LoadedLandblock(
     uint LandblockId,
     LandBlock Heightmap,
-    IReadOnlyList<WorldEntity> Entities);
+    IReadOnlyList<WorldEntity> Entities,
+    PhysicsDatBundle? PhysicsDats = null);