- Cache stamp sounds in stamp_system.gd and StampBarController.gd
  to avoid loading audio from disk on every stamp action
- Cache missile textures in PotatoFactory.gd to avoid reloading
  on every missile creation during border runner gameplay
- Reuse Timer in juicy_buttons.gd hover handler instead of
  creating/destroying on every mouse enter/exit
- Use threaded scene loading in ShiftSummaryScreen.gd for
  smoother viewport transitions