Remove entity self-reference cycle

UIEntity::init() stored self->data->self = (PyObject*)self with
Py_INCREF(self), creating a reference cycle that prevented entities
from ever being freed. The matching Py_DECREF never existed.

Fix: Remove the `self` field from UIEntity entirely. Replace all
read sites (iter next, getitem, get_perspective, entities_in_radius)
with PythonObjectCache lookups using serial_number, which uses weak
references and doesn't prevent garbage collection.

Also adds tp_dealloc to PyUIEntityType to properly clean up the
shared_ptr and weak references when the Python wrapper is freed.

Closes #266, closes #275

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

tests/regression/issue_266_entity_lifecycle_test.py

@@ -0,0 +1,100 @@
+"""Regression test: entity self-reference cycle (#266, #275).
+
+Bug: UIEntity::init() stored a reference to the Python wrapper via
+self->data->self = (PyObject*)self and Py_INCREF(self). This created
+a reference cycle that prevented the entity from ever being freed,
+even after removing it from all grids and dropping all Python references.
+
+Fix: Remove the self field entirely. Use PythonObjectCache (weak refs)
+for Python object identity preservation instead.
+"""
+import mcrfpy
+import gc
+import sys
+
+def test_entity_accessible_via_grid():
+    """Entities remain accessible through grid.entities after Python ref dropped"""
+    grid = mcrfpy.Grid(grid_size=(10, 10))
+    entity = mcrfpy.Entity(grid_pos=(5, 5), grid=grid)
+    entity_id = id(entity)
+
+    # Drop the Python reference
+    del entity
+    gc.collect()
+
+    # Entity should still be accessible via grid
+    assert len(grid.entities) == 1, f"Expected 1 entity, got {len(grid.entities)}"
+    retrieved = grid.entities[0]
+    assert retrieved.grid_x == 5.0, f"Expected x=5.0, got {retrieved.grid_x}"
+    print("  PASS: entity_accessible_via_grid")
+
+def test_entity_removed_from_grid():
+    """Entity can be removed from grid cleanly"""
+    grid = mcrfpy.Grid(grid_size=(10, 10))
+    entity = mcrfpy.Entity(grid_pos=(3, 3), grid=grid)
+
+    assert len(grid.entities) == 1
+    grid.entities.remove(entity)
+    assert len(grid.entities) == 0
+    print("  PASS: entity_removed_from_grid")
+
+def test_multiple_entities():
+    """Multiple entities can be created, accessed, and removed"""
+    grid = mcrfpy.Grid(grid_size=(20, 20))
+    entities = []
+    for i in range(10):
+        e = mcrfpy.Entity(grid_pos=(i, i), grid=grid)
+        entities.append(e)
+
+    assert len(grid.entities) == 10
+
+    # Drop all Python references
+    del entities
+    gc.collect()
+
+    # All should still be in grid
+    assert len(grid.entities) == 10
+
+    # Access each one
+    for i in range(10):
+        e = grid.entities[i]
+        assert e.grid_x == float(i), f"Entity {i} x={e.grid_x}, expected {float(i)}"
+
+    print("  PASS: multiple_entities")
+
+def test_entity_transfer_preserves_identity():
+    """Entity identity preserved when transferring between grids"""
+    grid1 = mcrfpy.Grid(grid_size=(10, 10))
+    grid2 = mcrfpy.Grid(grid_size=(20, 20))
+    entity = mcrfpy.Entity(grid_pos=(5, 5), grid=grid1)
+
+    entity.grid = grid2
+    assert len(grid1.entities) == 0
+    assert len(grid2.entities) == 1
+
+    retrieved = grid2.entities[0]
+    assert retrieved.grid_x == 5.0
+    print("  PASS: entity_transfer_preserves_identity")
+
+def test_iteration_after_gc():
+    """Iterating grid.entities works after GC of Python wrappers"""
+    grid = mcrfpy.Grid(grid_size=(10, 10))
+    for i in range(5):
+        mcrfpy.Entity(grid_pos=(i, 0), grid=grid)
+
+    gc.collect()
+
+    count = 0
+    for e in grid.entities:
+        count += 1
+    assert count == 5, f"Expected 5 entities in iteration, got {count}"
+    print("  PASS: iteration_after_gc")
+
+print("Testing entity lifecycle (self-reference removal)...")
+test_entity_accessible_via_grid()
+test_entity_removed_from_grid()
+test_multiple_entities()
+test_entity_transfer_preserves_identity()
+test_iteration_after_gc()
+print("PASS: all entity lifecycle tests passed")
+sys.exit(0)