feat: Implement Phase A UI hierarchy foundations (closes #122, #102, #116, #118)

Parent-Child UI System (#122):
- Add parent weak_ptr to UIDrawable for hierarchy tracking
- Add setParent(), getParent(), removeFromParent() methods
- UICollection now tracks owner and sets parent on append/insert
- Auto-remove from old parent when adding to new collection

Global Position Property (#102):
- Add get_global_position() that walks up parent chain
- Expose as read-only 'global_position' property on all UI types
- Add UIDRAWABLE_PARENT_GETSETTERS macro for consistent bindings

Dirty Flag System (#116):
- Modify markDirty() to propagate up the parent chain
- Add isDirty() and clearDirty() methods for render optimization

Scene as Drawable (#118):
- Add position, visible, opacity properties to Scene
- Add setProperty()/getProperty() for animation support
- Apply scene transformations in PyScene::render()
- Fix lifecycle callbacks to clear errors when methods don't exist
- Add GameEngine::getScene() public accessor

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

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

src/UICircle.cpp

@@ -369,6 +369,7 @@ PyGetSetDef UICircle::getsetters[] = {
     {"pos", (getter)UIDrawable::get_pos, (setter)UIDrawable::set_pos,
      "Position as a Vector (same as center).", (void*)PyObjectsEnum::UICIRCLE},
     UIDRAWABLE_GETSETTERS,
+    UIDRAWABLE_PARENT_GETSETTERS(PyObjectsEnum::UICIRCLE),
     {NULL}
 };