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/Scene.cpp

@@ -54,3 +54,43 @@ void Scene::key_unregister()
     */
     key_callable.reset();
 }
+
+// #118: Scene animation property support
+bool Scene::setProperty(const std::string& name, float value)
+{
+    if (name == "x") {
+        position.x = value;
+        return true;
+    }
+    if (name == "y") {
+        position.y = value;
+        return true;
+    }
+    if (name == "opacity") {
+        opacity = std::max(0.0f, std::min(1.0f, value));
+        return true;
+    }
+    if (name == "visible") {
+        visible = (value != 0.0f);
+        return true;
+    }
+    return false;
+}
+
+bool Scene::setProperty(const std::string& name, const sf::Vector2f& value)
+{
+    if (name == "pos" || name == "position") {
+        position = value;
+        return true;
+    }
+    return false;
+}
+
+float Scene::getProperty(const std::string& name) const
+{
+    if (name == "x") return position.x;
+    if (name == "y") return position.y;
+    if (name == "opacity") return opacity;
+    if (name == "visible") return visible ? 1.0f : 0.0f;
+    return 0.0f;
+}