Implement Scene subclass on_key callback support

Scene subclasses can now define on_key(self, key, state) methods that
receive keyboard events, matching the existing on_enter, on_exit, and
update lifecycle callbacks.

Changes:
- Rename call_on_keypress to call_on_key (consistent naming with property)
- Add triggerKeyEvent helper in McRFPy_API
- Call triggerKeyEvent from GameEngine when key_callable is not set
- Fix condition to check key_callable.isNone() (not just pointer existence)
- Handle both bound methods and instance-assigned callables

Usage:
    class GameScene(mcrfpy.Scene):
        def on_key(self, key, state):
            if key == "Escape" and state == "end":
                quit_game()

Property assignment (scene.on_key = callable) still works and takes
precedence when key_callable is set via the property setter.

Includes comprehensive test: tests/unit/scene_subclass_on_key_test.py

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/McRFPy_API.h

@@ -80,6 +80,7 @@ public:
     static void triggerSceneChange(const std::string& from_scene, const std::string& to_scene);
     static void updatePythonScenes(float dt);
     static void triggerResize(int width, int height);
+    static void triggerKeyEvent(const std::string& key, const std::string& action);
 
     // #151: Module-level scene property accessors
     static PyObject* api_get_current_scene();