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>

tests/regression/issue_253_spatial_hash_test.py

@@ -0,0 +1,73 @@
+"""Regression test for #253: GridPoint.entities uses spatial hash for O(1) lookup."""
+import mcrfpy
+import sys
+
+def test_gridpoint_entities_basic():
+    """Entities at known positions are returned correctly."""
+    scene = mcrfpy.Scene("test253")
+    mcrfpy.current_scene = scene
+
+    tex = mcrfpy.Texture("assets/kenney_tinydungeon.png", 16, 16)
+    grid = mcrfpy.Grid(grid_size=(20, 20), texture=tex, pos=(0, 0), size=(320, 320))
+    scene.children.append(grid)
+
+    # Place entities at specific cells
+    e1 = mcrfpy.Entity((5, 5), grid=grid)
+    e2 = mcrfpy.Entity((5, 5), grid=grid)
+    e3 = mcrfpy.Entity((10, 10), grid=grid)
+
+    # Query cell (5, 5) - should have 2 entities
+    cell_5_5 = grid.at(5, 5)
+    ents = cell_5_5.entities
+    assert len(ents) == 2, f"Expected 2 entities at (5,5), got {len(ents)}"
+    print("PASS: 2 entities at (5,5)")
+
+    # Query cell (10, 10) - should have 1 entity
+    cell_10_10 = grid.at(10, 10)
+    ents = cell_10_10.entities
+    assert len(ents) == 1, f"Expected 1 entity at (10,10), got {len(ents)}"
+    print("PASS: 1 entity at (10,10)")
+
+def test_gridpoint_entities_empty():
+    """Empty cells return empty list."""
+    scene = mcrfpy.Scene("test253b")
+    mcrfpy.current_scene = scene
+
+    tex = mcrfpy.Texture("assets/kenney_tinydungeon.png", 16, 16)
+    grid = mcrfpy.Grid(grid_size=(20, 20), texture=tex, pos=(0, 0), size=(320, 320))
+    scene.children.append(grid)
+
+    # No entities placed - empty cell should return empty list
+    cell = grid.at(0, 0)
+    ents = cell.entities
+    assert len(ents) == 0, f"Expected 0 entities, got {len(ents)}"
+    print("PASS: empty cell returns empty list")
+
+def test_gridpoint_entities_after_move():
+    """Moving an entity updates spatial hash so GridPoint.entities reflects new position."""
+    scene = mcrfpy.Scene("test253c")
+    mcrfpy.current_scene = scene
+
+    tex = mcrfpy.Texture("assets/kenney_tinydungeon.png", 16, 16)
+    grid = mcrfpy.Grid(grid_size=(20, 20), texture=tex, pos=(0, 0), size=(320, 320))
+    scene.children.append(grid)
+
+    e = mcrfpy.Entity((3, 3), grid=grid)
+
+    # Verify entity is at (3, 3)
+    assert len(grid.at(3, 3).entities) == 1, "Entity should be at (3,3)"
+
+    # Move entity to (7, 7)
+    e.grid_pos = (7, 7)
+
+    # Old cell should be empty, new cell should have the entity
+    assert len(grid.at(3, 3).entities) == 0, "Old cell should be empty after move"
+    assert len(grid.at(7, 7).entities) == 1, "New cell should have entity after move"
+    print("PASS: entity move updates spatial hash correctly")
+
+if __name__ == "__main__":
+    test_gridpoint_entities_basic()
+    test_gridpoint_entities_empty()
+    test_gridpoint_entities_after_move()
+    print("All #253 tests passed")
+    sys.exit(0)