feat(app): UCG W2 Task 2 — render root from physics CurrCell (FindCameraCell fallback)

Wire the BFS visibility root to DataCache.CellGraph.CurrCell (the physics
membership answer written in W2 Task 1) rather than resolving independently
from a position via FindCameraCell.  Closes the render/physics disagreement
that causes the "world from below" spawn-in flicker.

Changes:
- CellVisibility.GetVisibleCells: extracted BFS body into new private
  GetVisibleCellsFromRoot(LoadedCell root, Vector3 cameraPos); existing
  GetVisibleCells delegates to it after FindCameraCell (behavior unchanged).
- CellVisibility.ComputeVisibilityFromRoot(LoadedCell? root, Vector3 fallbackPos):
  new public entry point; when root is null falls through to ComputeVisibility
  (exact today's behavior), otherwise sets _lastCameraCell = root and delegates
  to GetVisibleCellsFromRoot — cannot regress below baseline.
- GameWindow (line 7156): replaced ComputeVisibility(visRootPos) with
  ComputeVisibilityFromRoot(physicsRoot, visRootPos) where physicsRoot is
  resolved from _physicsEngine.DataCache.CellGraph.CurrCell via TryGetCell.
  physicsRoot is null whenever CurrCell is null or its id is not yet in the
  render registry, so the fallback fires until the cell loads.
- 6 new tests in CellVisibilityFromRootTests: null-root fallback equivalence
  (3 cases), registered root → CameraCell == root (3 cases).  All 160 App.Tests
  pass, 0 regressions.

Visual verification PENDING — behavior change; do not claim it works visually.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/AcDream.App/Rendering/GameWindow.cs

@@ -7153,7 +7153,17 @@ public sealed class GameWindow : IDisposable
             var visRootPos = (_playerMode && _playerController is not null)
                 ? _playerController.Position
                 : camPos;
-            var visibility = _cellVisibility.ComputeVisibility(visRootPos);
+            // UCG W2: use the physics membership answer (DataCache.CellGraph.CurrCell) as the
+            // BFS root instead of resolving from position via FindCameraCell. Falls back to the
+            // original ComputeVisibility path when the physics answer isn't usable yet (null
+            // CurrCell, or its cell id not yet registered with the render CellVisibility system).
+            // This closes the render/physics disagreement — both now key off the same BSP-based
+            // resolution — which is the root cause of the "world from below" spawn flicker.
+            LoadedCell? physicsRoot = null;
+            if (_physicsEngine.DataCache?.CellGraph.CurrCell is AcDream.Core.World.Cells.EnvCell physCell
+                && _cellVisibility.TryGetCell(physCell.Id, out var registeredCell))
+                physicsRoot = registeredCell;
+            var visibility = _cellVisibility.ComputeVisibilityFromRoot(physicsRoot, visRootPos);
             bool cameraInsideCell = visibility?.CameraCell is not null;
 
             // Phase U.4 (2026-05-30): the [vis] probe moved DOWN to the unified