feat(A.5 T7): LandblockStreamResult.Loaded.Tier+MeshData; Promoted variant

Extends the Loaded result record with a LandblockStreamTier discriminator
and a LandblockMeshData payload (default! stub — T13 wires the real
off-thread mesh build). Adds the Promoted variant for Far→Near upgrades
that only need the entity layer, not a mesh rebuild.

LandblockStreamer.HandleJob passes Tier.Near + default! MeshData at the
existing synchronous load site; StreamingControllerTests updated to
match the new positional signature.

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

src/AcDream.App/Streaming/LandblockStreamer.cs

@@ -169,8 +169,12 @@ public sealed class LandblockStreamer : IDisposable
                         _outbox.Writer.TryWrite(new LandblockStreamResult.Failed(
                             load.LandblockId, "LandblockLoader.Load returned null"));
                     else
+                        // TEMPORARY: passes default! for MeshData — Task 13 wires the real mesh build.
                         _outbox.Writer.TryWrite(new LandblockStreamResult.Loaded(
-                            load.LandblockId, lb));
+                            load.LandblockId,
+                            LandblockStreamTier.Near,
+                            lb,
+                            MeshData: default!  /* TODO(A.5 T13) */));
                 }
                 catch (Exception ex)
                 {