v0.2.1

Added

• The piano's own song database. async_get_song_db fetches /ctrl/song.json — the
  controller UI's backing store — parsed into SongDatabase / LibrarySong and cached on
  the client. It carries what the open API's listings omit, most usefully each song's media
  format, plus genre, composer, performer and length. async_lookup_song joins the
  sequencer's song_pfix/song_id pair (now exposed as MasterState.song_prefix /
  song_id) against the database, refreshing the cache once on a miss so fresh recordings
  and re-indexed shares are found.
• SongFormat, the database's media-format vocabulary, with the business rule attached:
  SongFormat.has_audio (and LibrarySong.has_audio) says whether playback uses the
  speaker path — recorded backing tracks (SMF,WAV, SMF,MP3, WAV) and XG-scored
  accompaniment (SMFXG) do; SMFSOLO and plain SMF drive only the keys. This is the
  same field the controller UI uses for its format badges and for locking the tempo control
  on audio-driven songs.
• Title search that returns candidates. async_search ranks songs, playlists and radio
  channels — exact over prefix over substring over fuzzy — and returns SearchResult rows
  ready to hand to the matching play call. The open API's search_title can only play its
  single fuzzy pick; this searches the song database instead, covering every library in one
  fetch, and treats a region without DisklavierRadio as simply contributing no results.
• The song database read gets its own response ceiling, MAX_SONG_DB_BYTES (8 MB): the
  database is most of a megabyte on a two-thousand-song unit, which the general 1 MB
  ceiling would already reject. Every other endpoint keeps the tighter bound.