fix(rig): the other two thirds of the gig-log fix, which 30fceb43's message claimed but did not contain

30fceb43 said the debt was "paid in three places" and shipped one file, the unit
itself. The inventory entry, the regenerated target and the hard gate stayed in
the working tree — so the commit that explains the fix and the commit that
performs it were different commits, and only one of them existed.

What is here:

  rig_units.py    gig-log joins SERVICES, so --ensure enables it and --status
                  reports it. 7 units, 0 problems. It was absent until now, and
                  that absence is the entire reason CosmicFest has no log: the
                  unit was installed 2026-07-29, never enabled, and nothing in
                  the authored list knew it existed.

  parvagues-rig.target  regenerated, now Wants 8 units.

  gig-up.sh       a HARD gate, deliberately not "is the unit active". A green
                  unit is not a recording — the process can be up while the
                  writer is wedged. It asserts a session file's mtime is within
                  20 s, i.e. it is writing NOW. Proved it can fail rather than
                  assuming: stopped gig-log, waited 25 s, gate failed at age
                  25s; restarted, age 1 s, passed.

Same class of error as the amend on c628415e earlier this session, where a message
described a midi-autoconnect fix the commit did not carry. Both times the message
was written from intent rather than from `git show --stat`.