Phase 1: Safety & performance foundation for Grid/Entity overhaul

- Fix Entity3D self-reference cycle: replace raw `self` pointer with
  `pyobject` strong-ref pattern matching UIEntity (closes #266)
- TileLayer inherits Grid texture when none set, in all three attachment
  paths: constructor, add_layer(), and .grid property (closes #254)
- Add SpatialHash::queryCell() for O(1) entity-at-cell lookup; fix
  missing spatial_hash.insert() in Entity.__init__ grid= kwarg path;
  use queryCell in GridPoint.entities (closes #253)
- Add FOV dirty flag and parameter cache to skip redundant computeFOV
  calls when map unchanged and params match (closes #292)

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

src/SpatialHash.h

@@ -33,6 +33,10 @@ public:
     // This removes from old bucket and inserts into new bucket if needed
     void update(std::shared_ptr<UIEntity> entity, float old_x, float old_y);
 
+    // Query all entities at a specific cell (exact integer position match)
+    // O(n) where n = entities in the bucket containing this cell
+    std::vector<std::shared_ptr<UIEntity>> queryCell(int x, int y) const;
+
     // Query all entities within radius of a point
     // Returns entities whose positions are within the circular radius
     std::vector<std::shared_ptr<UIEntity>> queryRadius(float x, float y, float radius) const;