fix: Make UICollection/EntityCollection match Python list semantics

Breaking change: UICollection.remove() now takes a value (element) instead
of an index, matching Python's list.remove() behavior.

New methods added to both UICollection and EntityCollection:
- pop([index]) -> element: Remove and return element at index (default: last)
- insert(index, element): Insert element at position

Semantic fixes:
- remove(element): Now removes first occurrence of element (was: remove by index)
- All methods now have docstrings documenting behavior

Note on z_index sorting: The collections are sorted by z_index before each
render. Using index-based operations (pop, insert) with non-default z_index
values may produce unexpected results. Use name-based .find() for stable
element access when z_index sorting is in use.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

src/UICollection.h

@@ -30,6 +30,8 @@ public:
 	static PyObject* append(PyUICollectionObject* self, PyObject* o);
 	static PyObject* extend(PyUICollectionObject* self, PyObject* iterable);
 	static PyObject* remove(PyUICollectionObject* self, PyObject* o);
+	static PyObject* pop(PyUICollectionObject* self, PyObject* args);
+	static PyObject* insert(PyUICollectionObject* self, PyObject* args);
 	static PyObject* index_method(PyUICollectionObject* self, PyObject* value);
 	static PyObject* count(PyUICollectionObject* self, PyObject* value);
 	static PyObject* find(PyUICollectionObject* self, PyObject* args, PyObject* kwds);