perf-audio:565 does the same chrt -f -p 80 on Ardour that parvagues-protect
does, plus 573 for every child at 79, and perf-watch reasserts it on any
drift by itself -- the Bridge journal caught it doing exactly that at
09:20:15 on Sep 23. So an AC that only tests the pv-protect edit passes at
home and fails at the venue.

Also marks the epistemic line, because the next reader will act on this
hours before a gig: the xrun counts and their attribution are measured, the
kill mechanism is consistent with every measurement but was never
reproduced, and it cannot be reproduced on demand -- it needs one >200ms
uninterrupted GUI burst. What makes the xrun half plausible is that
sched_rt_runtime_us sits at the default 950000/1000000, so RT tasks passing
95% of a period get every RT task on that runqueue throttled for the
remaining 50ms, which is 2.3 quanta at 1024/48k.