SYNC mode was Linux-only: it shelled out to parec. On macOS there is no
PulseAudio, so the analyser always fell back to the drifting stream tap.
Capture with 'ffmpeg -f avfoundation' from a loopback input instead
(BlackHole, Loopback, Soundflower, Multi-Output/Aggregate devices are
auto-detected; SOMAFM_SPECTRUM_DEVICE accepts a name or index). Plain
microphones are never auto-selected.
A loopback that nothing is routed into delivers endless digital silence,
so treat ~6 s of all-zero blocks as a dead monitor and fall back to the
stream tap rather than showing frozen bars. parec handling is unchanged.
beefcarver
69b46b9035