The argv-only bug is fixed by extracting the 3-rung ladder into
tools/ardour_session.py, imported by both gig-log and check-mix. gig-log's
'keep check-mix in sync' comment is gone because there is nothing left to keep
in sync — one parser per concept satisfied by construction rather than by
somebody remembering. 169 gig-log tests pass unchanged.

Then it ran for real and reported Tidal 07, 08, 10 and 12 at -inf dB in the
saved session. Third recurrence of that bug (04-08+12, then 02+10, now these
four). PLN's parked two-minute check went from unrunnable straight to failing,
which is the whole argument for wiring a check into a caller: it gets run under
conditions the unit test never sees.

Also logged: two pre-existing test failures the suite run made visible
(test_orphan_orbits 16 rows vs 13 expected; test_lcxl3_display collects zero
tests, and a test file that cannot fail is not a passing test), and the
uncommitted duplicates left sitting on claude/rig-streamline so whoever rebases
it does not double-apply them.

    the fader was down in the file, not in the room.
    nobody could hear the difference
    because a closed fader and a silent instrument
    look identical on a post-fader meter.