- Add position member to UIDrawable base class
- Add common position getters/setters (x, y, pos) to base class
- Update UIFrame to use base class position instead of box position
- Synchronize box position with base class position for rendering
- Update all UIFrame methods to use base position consistently
- Add comprehensive test coverage for UIFrame position handling
This is part 1 of moving position to the base class. Other derived classes
(UICaption, UISprite, UIGrid) will be updated in subsequent commits.
