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/CellVisibility.cs

@@ -263,6 +263,32 @@ public sealed class CellVisibility
         _cellLookup[cell.CellId] = cell;
     }
 
+    /// <summary>
+    /// Atomically replaces one landblock's complete portal-cell set on the
+    /// render thread. Streaming workers build the input privately; no partially
+    /// hydrated landblock is ever visible to the per-frame flood.
+    /// </summary>
+    /// <param name="landblockId">Full landblock id, e.g. 0xA9B4FFFF.</param>
+    public void CommitLandblock(uint landblockId, IReadOnlyList<LoadedCell> cells)
+    {
+        uint prefix = landblockId >> 16;
+        if (cells.Any(cell => (cell.CellId >> 16) != prefix))
+            throw new ArgumentException(
+                "A visibility cell belongs to a different landblock.",
+                nameof(cells));
+
+        if (_cellsByLandblock.TryGetValue(prefix, out var previous))
+        {
+            foreach (var cell in previous)
+                _cellLookup.Remove(cell.CellId);
+        }
+
+        var committed = new List<LoadedCell>(cells);
+        _cellsByLandblock[prefix] = committed;
+        foreach (var cell in committed)
+            _cellLookup[cell.CellId] = cell;
+    }
+
     /// <summary>
     /// Phase A8 (2026-05-28): enumerates the loaded cells that belong to a
     /// landblock prefix. Used by <c>GameWindow.ApplyLoadedTerrainLocked</c> when