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/LandblockBuild.cs

@@ -0,0 +1,16 @@
+using AcDream.App.Rendering.Wb;
+using AcDream.Core.World;
+
+namespace AcDream.App.Streaming;
+
+/// <summary>
+/// Complete CPU-side output of one streaming job. The worker owns this object
+/// until it posts the corresponding completion; the render thread then applies
+/// the landblock and its cell transaction together.
+/// </summary>
+public sealed record LandblockBuild(
+    LoadedLandblock Landblock,
+    EnvCellLandblockBuild? EnvCells = null)
+{
+    public uint LandblockId => Landblock.LandblockId;
+}