Drop background-music tracks here as MP3, named by theme key:
  menu.mp3        — home/menu (SeasonTheme.fallback, bgm="menu")
  season_001.mp3  — Season 1 "First Bloom"
  season_002.mp3  — Season 2 "Summer Tide"

Use CC0 / royalty-free, commercial-safe tracks (see docs). The app plays
whatever is present and stays silent (no error) for any missing track.
