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>

tests/AcDream.Core.Tests/World/PhysicsDatBundleTests.cs

@@ -0,0 +1,18 @@
+using AcDream.Core.World;
+using Xunit;
+
+namespace AcDream.Core.Tests.World;
+
+public class PhysicsDatBundleTests
+{
+    [Fact]
+    public void Empty_ReturnsNullInfoAndEmptyMaps()
+    {
+        var b = PhysicsDatBundle.Empty;
+        Assert.Null(b.Info);
+        Assert.Empty(b.EnvCells);
+        Assert.Empty(b.Environments);
+        Assert.Empty(b.Setups);
+        Assert.Empty(b.GfxObjs);
+    }
+}