feat: Add Entity.die() method for lifecycle management closes #30

- Remove entity from its grid's entity list
- Clear grid reference after removal
- Safe to call multiple times (no-op if not on grid)
- Works with shared_ptr entity management

src/UIEntity.h

@@ -53,6 +53,7 @@ public:
     
     static PyObject* at(PyUIEntityObject* self, PyObject* o);
     static PyObject* index(PyUIEntityObject* self, PyObject* Py_UNUSED(ignored));
+    static PyObject* die(PyUIEntityObject* self, PyObject* Py_UNUSED(ignored));
     static int init(PyUIEntityObject* self, PyObject* args, PyObject* kwds);
 
     static PyObject* get_position(PyUIEntityObject* self, void* closure);