fix(streaming): commit EnvCell landblocks atomically (#214)

Carry one complete cell payload with each streaming completion and publish visibility, physics, and render state on the render thread. Remove the obsolete post-readiness login reload that triggered a duplicate dungeon build.

Co-Authored-By: Codex <noreply@openai.com>

src/AcDream.App/Streaming/LandblockStreamJob.cs

@@ -42,9 +42,21 @@ public abstract record LandblockStreamResult(uint LandblockId)
     public sealed record Loaded(
         uint LandblockId,
         LandblockStreamTier Tier,
-        LoadedLandblock Landblock,
+        LandblockBuild Build,
         LandblockMeshData MeshData
-    ) : LandblockStreamResult(LandblockId);
+    ) : LandblockStreamResult(LandblockId)
+    {
+        public Loaded(
+            uint landblockId,
+            LandblockStreamTier tier,
+            LoadedLandblock landblock,
+            LandblockMeshData meshData)
+            : this(landblockId, tier, new LandblockBuild(landblock), meshData)
+        {
+        }
+
+        public LoadedLandblock Landblock => Build.Landblock;
+    }
 
     /// <summary>
     /// A previously-Far-resident landblock was promoted to Near. The result
@@ -56,10 +68,19 @@ public abstract record LandblockStreamResult(uint LandblockId)
     /// </summary>
     public sealed record Promoted(
         uint LandblockId,
-        LoadedLandblock Landblock,
+        LandblockBuild Build,
         LandblockMeshData MeshData
     ) : LandblockStreamResult(LandblockId)
     {
+        public Promoted(
+            uint landblockId,
+            LoadedLandblock landblock,
+            LandblockMeshData meshData)
+            : this(landblockId, new LandblockBuild(landblock), meshData)
+        {
+        }
+
+        public LoadedLandblock Landblock => Build.Landblock;
         public IReadOnlyList<WorldEntity> Entities => Landblock.Entities;
     }