feat(render): portal-based EnvCell visibility (Step 4)

Port ACME's EnvCellManager portal visibility system:

- New CellVisibility class: BFS portal traversal from camera cell,
  portal-side clip-plane test, FindCameraCell with grace period
- LoadedCell data populated during streaming (portals, clip planes,
  world/inverse transforms, local AABB from CellStruct vertices)
- WorldEntity.ParentCellId tags interior entities for filtering
- InstancedMeshRenderer.Draw accepts optional visibleCellIds set —
  interior entities whose parent cell isn't visible are skipped
- Conditional depth clear between terrain and static mesh when
  camera is inside a cell (ACME GameScene.cs pattern)

When camera is outdoors, all interiors render (visibleCellIds=null).
When camera enters a building, only BFS-reachable cells render.

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

src/AcDream.Core/World/WorldEntity.cs

@@ -35,4 +35,12 @@ public sealed class WorldEntity
     /// and team colors. Non-palette-indexed textures ignore this field.
     /// </summary>
     public PaletteOverride? PaletteOverride { get; init; }
+
+    /// <summary>
+    /// EnvCell ID that owns this entity (room geometry or static object inside
+    /// the cell). Used by portal visibility to filter interior entities — only
+    /// entities whose ParentCellId appears in the visible set are rendered.
+    /// Null for outdoor entities (stabs, scenery, live server spawns).
+    /// </summary>
+    public uint? ParentCellId { get; init; }
 }