Refactor text rendering subsystem
- Fully remove Render dependency from text rendering.
- Make Label own TextBuf and draw text in uiPass via Render::UiIn.
- Add Label::setText(), setColor(), and setSz().
- Update checkbox, fps-label, menu-item, stats-ui, tab, and related UI code to use the new Label API.
- Fully remove FontManager class.
- Delete font-type.hpp.
- Convert Atlas to a singleton.
- Rename cube-atlas.{hpp,cpp} to atlas.{hpp,cpp}.
- Add Atlas::init() and Atlas::shutdown(), called from BgfxInit.
- Font loads TTF data directly and caches SizedFont per pixel size.
- Extract TrueTypeFont from the old font-manager code.
- Add sized-font.{hpp,cpp} for sized font instances and glyph lookup.
- Remove text-metrics and move dimension logic into Font::getDimensions.