Master is a bus. It has no rec-enable, so Ardour can never record it, so an
unarmed orbit has nothing to fall back to — the whole cost of the 24th. The
answer inside Ardour is a `Mix` TRACK fed from Master/audio_out 1+2, armed:
stem grade, same take, same sample clock as the orbits.

`--mix` audits it, and exits 4 when there is no such route, because absence
is a choice and the gate must tell "you have not built the net" apart from
"your net is broken". Absent prints the 30-second Add Track recipe rather
than nagging. Once it exists, four things make it a net that looks present
and records nothing, and they are checked in the order they bite:

  1. output wired back to Master — a unity-gain FEEDBACK LOOP through the PA,
     which is the state you get by following Ardour's Add Track dialog and
     stopping, since a new track goes to Master by default;
  2. not armed — the Tidal 08 failure one route further out;
  3. input not on Master, or only half on it — one channel the mix and the
     other an orbit reads as a healthy file and is unusable;
  4. fader off unity while the route is DiskIOPostFader — the fader is IN the
     recording, and at -inf the file is digital silence.

Two gate probes, split the way the gig-record pair is: "mix track" ADVISES
that there is none, "mix track wired" BLOCKS on one that is broken.

Ten new tests. The mutated fixtures each break exactly one thing, so a
failure names the fault; ExtConnection is asserted NOT to count as an
in-session link, or a correctly wired track would report a feedback loop.