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/LandblockStreamJob.cs

@@ -1,3 +1,5 @@
+using System.Collections.Generic;
+using AcDream.Core.Terrain;
 using AcDream.Core.World;
 
 namespace AcDream.App.Streaming;
@@ -22,7 +24,29 @@ public abstract record LandblockStreamJob(uint LandblockId)
 /// </summary>
 public abstract record LandblockStreamResult(uint LandblockId)
 {
-    public sealed record Loaded(uint LandblockId, LoadedLandblock Landblock) : LandblockStreamResult(LandblockId);
+    /// <summary>
+    /// A landblock load completed. <see cref="Tier"/> distinguishes Far
+    /// (terrain only) from Near (terrain + entities). <see cref="MeshData"/>
+    /// is built off the render thread on the streaming worker.
+    /// </summary>
+    public sealed record Loaded(
+        uint LandblockId,
+        LandblockStreamTier Tier,
+        LoadedLandblock Landblock,
+        LandblockMeshData MeshData
+    ) : LandblockStreamResult(LandblockId);
+
+    /// <summary>
+    /// A previously-Far-resident landblock was promoted to Near. Terrain
+    /// mesh is already on the GPU; the result carries the entity layer
+    /// (stabs, buildings, scenery) to merge into the existing GpuWorldState
+    /// entry.
+    /// </summary>
+    public sealed record Promoted(
+        uint LandblockId,
+        IReadOnlyList<WorldEntity> Entities
+    ) : LandblockStreamResult(LandblockId);
+
     public sealed record Failed(uint LandblockId, string Error) : LandblockStreamResult(LandblockId);
     public sealed record Unloaded(uint LandblockId) : LandblockStreamResult(LandblockId);
 

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)
                 {

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

@@ -78,7 +78,7 @@ 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, lb));
+        fake.Pending.Enqueue(new LandblockStreamResult.Loaded(0x32320FFEu, LandblockStreamTier.Near, lb, MeshData: default!));
 
         controller.Tick(50, 50);