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/PySceneObject.h

@@ -36,7 +36,7 @@ public:
     // Lifecycle callbacks (called from C++)
     static void call_on_enter(PySceneObject* self);
     static void call_on_exit(PySceneObject* self);
-    static void call_on_keypress(PySceneObject* self, std::string key, std::string action);
+    static void call_on_key(PySceneObject* self, const std::string& key, const std::string& action);
     static void call_update(PySceneObject* self, float dt);
     static void call_on_resize(PySceneObject* self, int width, int height);
 
@@ -75,7 +75,7 @@ namespace mcrfpydef {
             "Lifecycle Callbacks (override in subclass):\n"
             "    on_enter(): Called when scene becomes active via activate().\n"
             "    on_exit(): Called when scene is deactivated (another scene activates).\n"
-            "    on_keypress(key: str, action: str): Called for keyboard events. Alternative to on_key property.\n"
+            "    on_key(key: str, action: str): Called for keyboard events (subclass method).\n"
             "    update(dt: float): Called every frame with delta time in seconds.\n"
             "    on_resize(width: int, height: int): Called when window is resized.\n\n"
             "Example:\n"