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/GameEngine.cpp

@@ -457,11 +457,17 @@ void GameEngine::processEvent(const sf::Event& event)
         std::string name = currentScene()->action(actionCode);
         currentScene()->doAction(name, actionType);
     }
-    else if (currentScene()->key_callable && 
+    else if (currentScene()->key_callable && !currentScene()->key_callable->isNone() &&
              (event.type == sf::Event::KeyPressed || event.type == sf::Event::KeyReleased))
     {
+        // Property-assigned handler (scene.on_key = callable)
         currentScene()->key_callable->call(ActionCode::key_str(event.key.code), actionType);
     }
+    else if (event.type == sf::Event::KeyPressed || event.type == sf::Event::KeyReleased)
+    {
+        // Try subclass on_key method if no property handler is set
+        McRFPy_API::triggerKeyEvent(ActionCode::key_str(event.key.code), actionType);
+    }
 }
 
 void GameEngine::sUserInput()