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/UIGrid.cpp

@@ -1499,6 +1499,7 @@ PyGetSetDef UIGrid::getsetters[] = {
      ), (void*)PyObjectsEnum::UIGRID},
     {"name", (getter)UIDrawable::get_name, (setter)UIDrawable::set_name, "Name for finding elements", (void*)PyObjectsEnum::UIGRID},
     UIDRAWABLE_GETSETTERS,
+    UIDRAWABLE_PARENT_GETSETTERS(PyObjectsEnum::UIGRID),
     {NULL}  /* Sentinel */
 };
 
@@ -1519,8 +1520,10 @@ PyObject* UIGrid::get_children(PyUIGridObject* self, void* closure)
     // Returns UICollection for UIDrawable children (speech bubbles, effects, overlays)
     auto type = (PyTypeObject*)PyObject_GetAttrString(McRFPy_API::mcrf_module, "UICollection");
     auto o = (PyUICollectionObject*)type->tp_alloc(type, 0);
+    Py_DECREF(type);
     if (o) {
         o->data = self->data->children;
+        o->owner = self->data;  // #122: Set owner for parent tracking
     }
     return (PyObject*)o;
 }