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/Animation.cpp

@@ -631,10 +631,10 @@ static PyObject* convertEntity3DToPython(std::shared_ptr<mcrf::Entity3D> entity)
         Py_RETURN_NONE;
     }
 
-    // Use the entity's cached Python self pointer if available
-    if (entity->self) {
-        Py_INCREF(entity->self);
-        return entity->self;
+    // Use the entity's cached Python pyobject pointer if available
+    if (entity->pyobject) {
+        Py_INCREF(entity->pyobject);
+        return entity->pyobject;
     }
 
     // Create a new wrapper