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);

src/AcDream.Core/World/PhysicsDatBundle.cs

@@ -0,0 +1,30 @@
+using System.Collections.Generic;
+using DatReaderWriter.DBObjs;
+// Environment collides with System.Environment under implicit usings — alias it.
+using DatEnvironment = DatReaderWriter.DBObjs.Environment;
+
+namespace AcDream.Core.World;
+
+/// <summary>
+/// The parsed dat objects <c>ApplyLoadedTerrainLocked</c> needs, pre-read by the
+/// streaming worker under <c>_datLock</c> so the apply makes ZERO DatCollection
+/// calls and the update thread never blocks on the worker's lock (the FPS
+/// 30↔200 swing was that lock-wait). Keyed by the same dat id the apply would
+/// have passed to <c>_dats.Get&lt;T&gt;</c>.
+/// </summary>
+public sealed record PhysicsDatBundle(
+    LandBlockInfo? Info,
+    IReadOnlyDictionary<uint, EnvCell> EnvCells,
+    IReadOnlyDictionary<uint, DatEnvironment> Environments,
+    IReadOnlyDictionary<uint, Setup> Setups,
+    IReadOnlyDictionary<uint, GfxObj> GfxObjs)
+{
+    /// <summary>Empty bundle for far-tier landblocks (no cells / buildings /
+    /// entities) and for non-streaming <see cref="LoadedLandblock"/> construction.</summary>
+    public static readonly PhysicsDatBundle Empty = new(
+        null,
+        new Dictionary<uint, EnvCell>(),
+        new Dictionary<uint, DatEnvironment>(),
+        new Dictionary<uint, Setup>(),
+        new Dictionary<uint, GfxObj>());
+}

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);
+    }
+}