fix(A.5 T10-T12): Start() race + null mesh test + real mesh stub

Code review on T10-T12 bundle (commits 0cf86bb/00bb030/0405947 + audit
fix 76e1a64) found 3 Important issues:

1. LandblockStreamer.Start() had an idempotency race — the XML doc
   claimed thread-safety but the implementation checked _worker != null
   before assigning, allowing two callers to both pass the check and
   spawn duplicate worker threads. Fixed via Interlocked.CompareExchange.

2. No test verified the worker emits Failed when buildMeshOrNull returns
   null. Added Load_WhenBuildMeshReturnsNull_ReportsFailed.

3. StreamingControllerTests.cs:81 used MeshData: default! when
   constructing a Loaded result. If a future test flows MeshData
   through the apply callback, the null reference would NRE rather
   than producing a meaningful assertion failure. Replaced with a real
   empty LandblockMeshData instance.

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

tests/AcDream.Core.Tests/Streaming/StreamingControllerTests.cs

@@ -78,7 +78,13 @@ public class StreamingControllerTests
         // Entities (positional record). Adjust if the first positional arg
         // name differs.
         var lb = new LoadedLandblock(0x32320FFEu, new LandBlock(), System.Array.Empty<WorldEntity>());
-        fake.Pending.Enqueue(new LandblockStreamResult.Loaded(0x32320FFEu, LandblockStreamTier.Near, lb, MeshData: default!));
+        // A.5 T10-T12 follow-up: use a real empty mesh instance instead of
+        // default! so any future test that flows MeshData through the apply
+        // callback gets a non-null reference to inspect rather than an NRE.
+        var stubMesh = new AcDream.Core.Terrain.LandblockMeshData(
+            System.Array.Empty<AcDream.Core.Terrain.TerrainVertex>(),
+            System.Array.Empty<uint>());
+        fake.Pending.Enqueue(new LandblockStreamResult.Loaded(0x32320FFEu, LandblockStreamTier.Near, lb, stubMesh));
 
         controller.Tick(50, 50);