* fix(cli): audio --read reports noise beds as ambience, not music
A stretch of the non-voice stems counted as music whenever it was above
-55 dB, so room tone and engine rumble separated into "other" printed as
a music piece. A stretch is now music only with a beat, clear tonal chroma
or more than one leading instrument; otherwise it is ambience with its
level in dBFS, called quiet under -45 dBFS, and breaks lie only between
two runs of music. The header lists only stems that carry sound, with
their level, instead of the separator's six fixed names.
* refactor(cli): audio --read gate carries its meaning in names, not comments
* fix(cli): keep ambience out of downstream music measurements
* fix(cli): measure music beat evidence within each stretch