feat(physics): complete retail collision — indoor BSP, dual sphere, step-up, swept-sphere, 6-path dispatcher

Indoor CellStruct PhysicsBSP collision for room walls/ceilings.
Dual sphere (body+head) from Setup dimensions.
StepUp attempts before sliding when hitting low obstacles.
FindTimeOfCollision for exact parametric BSP contact time.
Full 6-path BSP dispatcher wired into FindEnvCollisions.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/AcDream.App/Rendering/GameWindow.cs

@@ -1394,6 +1394,9 @@ public sealed class GameWindow : IDisposable
 
                         // Step 4: build LoadedCell for portal visibility.
                         BuildLoadedCell(envCellId, envCell, cellStruct, cellOrigin, cellTransform);
+
+                        // Cache CellStruct physics BSP for indoor collision.
+                        _physicsDataCache.CacheCellStruct(envCellId, cellStruct, cellTransform);
                     }
                 }
             }