UI START BACKGROUND NATIVE-SLOT FIX

When Hide original BG2 START UI is enabled:
- Runtime UI.MENU now references mosaic 'START'.
- The selected replacement image is exported/installed as override\START.MOS.
- The project Background MOS resref remains available for custom/non-START menus.
- This deliberately overrides BG2EE's stock START resource using the normal override precedence.
