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/Rendering/Wb/EnvCellMeshPreparationScheduler.cs

@@ -0,0 +1,23 @@
+namespace AcDream.App.Rendering.Wb;
+
+/// <summary>
+/// Starts CPU mesh extraction for a completed EnvCell build without publishing
+/// that build to the live renderer. ObjectMeshManager owns its thread-safe work
+/// queue; the render-thread commit remains a small atomic state replacement.
+/// </summary>
+public static class EnvCellMeshPreparationScheduler
+{
+    public static void Schedule(
+        EnvCellLandblockBuild build,
+        ObjectMeshManager meshManager)
+    {
+        foreach (var shell in build.Shells)
+        {
+            _ = meshManager.PrepareEnvCellGeomMeshDataAsync(
+                shell.GeometryId,
+                shell.EnvironmentId,
+                shell.CellStructure,
+                new List<ushort>(shell.Surfaces));
+        }
+    }
+}