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>

tests/AcDream.App.Tests/Rendering/CornerFloodReplayTests.cs

@@ -18,7 +18,7 @@ namespace AcDream.App.Tests.Rendering;
 /// the neighbour room through the 0171↔0173↔0172 doorway chain. The user still sees
 /// BACKGROUND at certain angles — so the defect is in the FLOOD/CLIP output for those
 /// eye positions. This harness drives the REAL <see cref="PortalVisibilityBuilder.Build"/>
-/// over the REAL Holtburg building cells (dat-loaded, mirroring GameWindow.BuildLoadedCell)
+/// over the REAL Holtburg building cells (dat-loaded, mirroring EnvCellLandblockBuildBuilder)
 /// along the captured corner eye path, and prints which cells the flood keeps/drops per
 /// step. The player's room (0172) dropping while the player is visible on screen is the
 /// background defect; the step where it happens pins the mechanism.
@@ -50,7 +50,7 @@ public class CornerFloodReplayTests
     }
 
     /// <summary>
-    /// Dat → LoadedCell, mirroring GameWindow.BuildLoadedCell (GameWindow.cs:5636-5776)
+    /// Dat → LoadedCell, mirroring EnvCellLandblockBuildBuilder.
     /// field-for-field: portals (with OtherPortalId back-link), clip planes from the
     /// portal polygon's first 3 verts + centroid InsideSide, full portal polygons in
     /// cell-local space, local AABB, world transform from EnvCell.Position.
@@ -106,7 +106,7 @@ public class CornerFloodReplayTests
                 var normal = Vector3.Normalize(Vector3.Cross(p1 - p0, p2 - p0));
                 float d = -Vector3.Dot(normal, p0);
                 // InsideSide from the dat PortalSide bit — mirrors the production fix
-                // (GameWindow.BuildLoadedCell): retail InitCell (0x005a4b70) + physics
+                // (EnvCellLandblockBuildBuilder): retail InitCell (0x005a4b70) + physics
                 // CellTransit use the dat bit; the old AABB-centroid guess mis-sided thin
                 // connectors (#186). Kept identical here so this replay stays a faithful mirror.
                 clipPlanes.Add(new PortalClipPlane