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_292_fov_dedup_test.py

@@ -0,0 +1,118 @@
+"""Regression test for #292: Deduplicate FOV computation via dirty flag."""
+import mcrfpy
+import sys
+
+def test_fov_basic_correctness():
+    """FOV computation still works correctly with dirty flag."""
+    scene = mcrfpy.Scene("test292")
+    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)
+
+    # Make all cells transparent and walkable
+    for y in range(20):
+        for x in range(20):
+            pt = grid.at(x, y)
+            pt.walkable = True
+            pt.transparent = True
+
+    # Compute FOV from center
+    grid.compute_fov((10, 10), radius=5)
+
+    # Center should be visible
+    assert grid.is_in_fov((10, 10)), "Center should be in FOV"
+    # Nearby cell should be visible
+    assert grid.is_in_fov((10, 11)), "Adjacent cell should be in FOV"
+    # Far cell should NOT be visible
+    assert not grid.is_in_fov((0, 0)), "Far cell should not be in FOV"
+    print("PASS: FOV basic correctness")
+
+def test_fov_duplicate_call():
+    """Calling computeFOV twice with same params should still give correct results."""
+    scene = mcrfpy.Scene("test292b")
+    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)
+
+    for y in range(20):
+        for x in range(20):
+            pt = grid.at(x, y)
+            pt.walkable = True
+            pt.transparent = True
+
+    # Compute FOV twice with same params (second should be skipped internally)
+    grid.compute_fov((10, 10), radius=5)
+    result1 = grid.is_in_fov((10, 11))
+
+    grid.compute_fov((10, 10), radius=5)
+    result2 = grid.is_in_fov((10, 11))
+
+    assert result1 == result2, "Duplicate FOV call should give same result"
+    print("PASS: Duplicate FOV call gives same result")
+
+def test_fov_updates_after_map_change():
+    """FOV should recompute after a cell's walkable/transparent changes."""
+    scene = mcrfpy.Scene("test292c")
+    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)
+
+    for y in range(20):
+        for x in range(20):
+            pt = grid.at(x, y)
+            pt.walkable = True
+            pt.transparent = True
+
+    # Compute FOV - cell (10, 12) should be visible
+    grid.compute_fov((10, 10), radius=5)
+    assert grid.is_in_fov((10, 12)), "Cell should be visible initially"
+
+    # Block line of sight by making (10, 11) opaque
+    grid.at(10, 11).transparent = False
+
+    # Recompute FOV with same params - dirty flag should force recomputation
+    grid.compute_fov((10, 10), radius=5)
+    assert not grid.is_in_fov((10, 12)), "Cell behind wall should not be visible after map change"
+    print("PASS: FOV updates correctly after map change")
+
+def test_fov_different_params_recompute():
+    """FOV should recompute when params change even if map hasn't."""
+    scene = mcrfpy.Scene("test292d")
+    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)
+
+    for y in range(20):
+        for x in range(20):
+            pt = grid.at(x, y)
+            pt.walkable = True
+            pt.transparent = True
+
+    # Compute from (10, 10) with radius 3
+    grid.compute_fov((10, 10), radius=3)
+    visible_3 = grid.is_in_fov((10, 14))
+
+    # Compute from (10, 10) with radius 5 - different params should recompute
+    grid.compute_fov((10, 10), radius=5)
+    visible_5 = grid.is_in_fov((10, 14))
+
+    # Radius 3 shouldn't see (10, 14), but radius 5 should
+    assert not visible_3, "(10,14) should not be visible with radius 3"
+    assert visible_5, "(10,14) should be visible with radius 5"
+    print("PASS: Different FOV params force recomputation")
+
+if __name__ == "__main__":
+    test_fov_basic_correctness()
+    test_fov_duplicate_call()
+    test_fov_updates_after_map_change()
+    test_fov_different_params_recompute()
+    print("All #292 tests passed")
+    sys.exit(0)