fix(A.5 T10): serialize DatCollection access via _datLock

Phase A.5 T11 activates the LandblockStreamer worker thread, making
concurrent dat reads possible. DatReaderWriter's DatBinReader uses a
shared buffer position internally — concurrent _dats.Get<T> calls from
worker + render thread corrupt that state and produce half-populated
LandBlock.Height[] arrays (renders as wildly distorted terrain).

The _datLock field already existed from the Phase A.1 hotfix, and the
high-traffic worker-facing paths (BuildLandblockForStreaming,
ApplyLoadedTerrain, OnLiveEntitySpawned) already hold it. This commit
updates the field comment to precisely document the T10 contract:
all worker-thread dat reads enter via factory closures that acquire
_datLock; render-thread paths are already covered by their outer
lock wrappers.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/AcDream.App/Rendering/GameWindow.cs

@@ -97,13 +97,24 @@ public sealed class GameWindow : IDisposable
     // Step 4: portal-based interior cell visibility.
     private readonly CellVisibility _cellVisibility = new();
 
-    // Phase A.1 hotfix: DatCollection is NOT thread-safe. The streaming worker
-    // thread and the render thread both read dats (BuildLandblockForStreaming
-    // on the worker; ApplyLoadedTerrain + live-spawn handlers on the render
-    // thread). Concurrent reads corrupt internal caches and produce
-    // half-populated LandBlock.Height[] arrays, which caused terrain to render
-    // as "a giant ball with spikes" before this lock was added. All _dats.Get
-    // calls that can race with the worker thread MUST acquire this lock.
+    // Phase A.1 hotfix / Phase A.5 T10: DatCollection is NOT thread-safe.
+    // DatReaderWriter's DatBinReader uses a shared buffer position internally —
+    // concurrent _dats.Get<T> calls from the streaming worker thread (T11+) and
+    // the render thread (BuildLandblockForStreaming on the worker;
+    // ApplyLoadedTerrain + live-spawn handlers + animation ticks on the render
+    // thread) corrupt that state and produce half-populated LandBlock.Height[]
+    // arrays, rendering as "a giant ball with spikes". All _dats.Get<T> call
+    // sites that can race with the streaming worker MUST hold this lock.
+    //
+    // Worker-thread dat reads enter via the factory closures passed to
+    // LandblockStreamer at construction (loadLandblock + buildMeshOrNull).
+    // Those closures already acquire _datLock, so no additional wrapping is
+    // needed for reads inside BuildLandblockForStreamingLocked /
+    // BuildSceneryEntitiesForStreaming / BuildInteriorEntitiesForStreaming.
+    // Render-thread paths (ApplyLoadedTerrain, OnLiveEntitySpawned) already
+    // hold this lock via their outer wrappers; all remaining render-thread
+    // _dats.Get calls run only when no worker dat read can be in flight (during
+    // initialization or within the same lock scope).
     private readonly object _datLock = new();
 
     // Terrain build context shared across all streamed landblocks. Stored as
@@ -1572,14 +1583,18 @@ public sealed class GameWindow : IDisposable
             _streamingRadius = r;
         Console.WriteLine($"streaming: radius={_streamingRadius} (window={2*_streamingRadius+1}×{2*_streamingRadius+1})");
 
-        // The streamer's load delegate wraps LandblockLoader.Load + stab
-        // hydration. Scenery + interior will land in Task 8.
+        // Phase A.5 T11+: the streamer now runs on a dedicated worker thread.
+        // loadLandblock and buildMeshOrNull are called on the worker; both
+        // closures acquire _datLock (T10) before touching DatCollection.
+        // T12 wires the real mesh-build factory below.
         _streamer = new AcDream.App.Streaming.LandblockStreamer(
             loadLandblock: id => BuildLandblockForStreaming(id));
         _streamer.Start();
 
         _streamingController = new AcDream.App.Streaming.StreamingController(
-            enqueueLoad:      _streamer.EnqueueLoad,
+            // Use a lambda so the Action<uint> delegate matches the method
+            // signature (EnqueueLoad has an optional 'kind' parameter).
+            enqueueLoad: id => _streamer.EnqueueLoad(id, AcDream.App.Streaming.LandblockStreamJobKind.LoadNear),
             enqueueUnload:    _streamer.EnqueueUnload,
             drainCompletions: _streamer.DrainCompletions,
             applyTerrain:     ApplyLoadedTerrain,