master_sink() took the first thing Master was linked to, so a spectrogram tap
listed before the real sink pointed the recorder at a source that does not
exist: RUNNING while capturing nothing. Only alsa_output/bluez_output count.
It also quit awk at the first hit, so under pipefail a large pw-link graph
gave SIGPIPE (141) and killed 'check' and, by the same race, 'start'.
