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>
