feat: Add consistent Scene API with module-level properties (closes #151)

Replaces module-level scene functions with more Pythonic OO interface:

Scene class changes:
- Add `scene.children` property (replaces get_ui() method)
- Add `scene.on_key` getter/setter (matches on_click pattern)
- Remove get_ui() method

Module-level properties:
- Add `mcrfpy.current_scene` (getter returns Scene, setter activates)
- Add `mcrfpy.scenes` (read-only tuple of all Scene objects)

Implementation uses custom module type (McRFPyModuleType) inheriting
from PyModule_Type with tp_setattro for property assignment support.

New usage:
  scene = mcrfpy.Scene("game")
  mcrfpy.current_scene = scene
  scene.on_key = handler
  ui = scene.children

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

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

src/McRFPy_API.h

@@ -98,6 +98,11 @@ public:
     static void updatePythonScenes(float dt);
     static void triggerResize(int width, int height);
 
+    // #151: Module-level scene property accessors
+    static PyObject* api_get_current_scene();
+    static int api_set_current_scene(PyObject* value);
+    static PyObject* api_get_scenes();
+
     // Exception handling - signal game loop to exit on unhandled Python exceptions
     static std::atomic<bool> exception_occurred;
     static std::atomic<int> exit_code;