fix: prevent segfault when closing window via X button

- Add cleanup() method to GameEngine to clear Python references before destruction
- Clear timers and McRFPy_API references in proper order
- Call cleanup() at end of run loop and in destructor
- Ensure cleanup is only called once per GameEngine instance

Also includes:
- Fix audio ::stop() calls (already in place, OpenAL warning is benign)
- Add Caption support for x, y keywords (e.g. Caption("text", x=5, y=10))
- Refactor UIDrawable_methods.h into UIBase.h for better organization
- Move UIEntity-specific implementations to UIEntityPyMethods.h

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

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

src/UIGrid.cpp

@@ -3,7 +3,7 @@
 #include "McRFPy_API.h"
 #include "PyPositionHelper.h"
 #include <algorithm>
-#include "UIDrawable_methods.h"
+// UIDrawable methods now in UIBase.h
 
 UIGrid::UIGrid() 
 : grid_x(0), grid_y(0), zoom(1.0f), center_x(0.0f), center_y(0.0f), ptex(nullptr)