rememberAudioVizState

An AudioVizState that listens to player for as long as it stays in the composition.

Remember it next to the player rather than inside the audio-only branch, so the analysis is already running when a song starts. songScan decides which items a background scan may read to build a song map; see docs/audioviz-song-scan-api.md.

songMapStore keeps finished maps between runs, so a song played before is mapped from its first note instead of scanned again. Give it a directory of the application's own, with SongMapStore.inDirectory. Without one, a map lives only as long as the process.