refactor: Use property setter pattern for parent assignment

Instead of separate getParent()/setParent()/removeFromParent() methods,
the parent property now supports the Pythonic getter/setter pattern:
- child.parent       # Get parent (or None)
- child.parent = f   # Set parent (adds to f.children)
- child.parent = None # Remove from parent

This matches the existing pattern used by the click property callback.

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

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

src/UIDrawable.h

@@ -95,6 +95,7 @@ public:
 
     // Python API for parent/global_position
     static PyObject* get_parent(PyObject* self, void* closure);
+    static int set_parent(PyObject* self, PyObject* value, void* closure);
     static PyObject* get_global_pos(PyObject* self, void* closure);
     
     // New properties for Phase 1