gig: a venue checklist, and the tray is not the desktop icon

The relaunch settled the acceptance-test question by measurement: perf-tray.py hands Ardour RLIMIT_RTTIME=unlimited, so the tray path was never exposed to the 200ms kill that the desktop icon reproduces. On the tray the fix earns its keep on the xrun half instead, and that half verifies: AudioEngine at FIFO/83, nothing at 80, graph back at 50 links.

Two things could not be tested at home and go in writing instead: the UMC202HD, with both failure signatures this box has produced before, and Bluetooth off before the set - because the pre-fix Ardour had no realtime at all, and a WirePlumber restart under a running Ardour is the likeliest way to lose it.

TODO_GIG.md

@@ -1028,3 +1028,74 @@ what `armada/setlist_thu24.txt` was built for — 17 tracks, ~9 min each, settle
       than a stale desk — but `check-mix` reads the saved session and `use-osc=0`,
       so no tool can see the live desk. **One Ctrl+S settles it**, and it has to
       happen before the Ardour quit in step 2 or the live desk is lost.
+
+## 🚐 Thu 24, 11:10 — AT THE VENUE, in order
+
+The rig is green at home. `parvagues-protect --check` exits 0, the set compiles
+17/17, the preload warms 137/137. Two things could not be tested here.
+
+### 1. The UMC202HD, first thing, before Ardour
+
+It has **never been on this box**. Plug it in *before* launching Ardour, because
+**Ardour restores its own saved ports and does NOT follow the PipeWire default
+sink** — so the interface being present is not the interface being used.
+
+```
+tools/check-audio-graph.sh            # this is exactly what it is for
+pw-link -l | grep -A3 'ardour:Master' # is Master on the UMC, or still on the laptop?
+```
+
+Two failure signatures to recognise, both seen on this box before:
+- **UCM profile lies.** A card can enumerate with a profile reporting
+  `available: yes` unconditionally while nothing is plugged in. `tools/check-audio.py`
+  detects it and refuses a fix that would leave zero sinks.
+- **Ardour keeps the old ports.** Master still wired to
+  `alsa_output.pci-…sof_sdw…` instead of the UMC. Re-point it in Ardour, then
+  Ctrl+S *while the faders are where you play them*.
+
+### 2. Bluetooth OFF before the set
+
+New evidence this morning: the pre-fix Ardour (PID 615285) had **no realtime at
+all** — `AudioEngine 1` was plain `TS`. The one launched from the tray at 11:05
+has `SCHED_FIFO|SCHED_RESET_ON_FORK/83`, exactly as the fix's rationale claims.
+The likeliest way a running Ardour loses its RT grant *and* its links is a
+**PipeWire/WirePlumber restart underneath it** — which, per
+`project_wireplumber-churn-bt`, a phone and a BLE lamp reconnecting cause about
+once a minute. **0 restarts today**, but Bluetooth is powered on and the venue is
+full of phones.
+
+```
+systemctl --user restart wireplumber   # NEVER during the set: it kills every link
+bluetoothctl power off                 # do this instead, before you start
+```
+
+### 3. The relaunch question, settled
+
+**The ParVagues tray is not the desktop icon.** The tray launch's parent is
+`perf-tray.py`, and it inherits `RLIMIT_RTTIME=unlimited` — same as the Bridge.
+The desktop icon inherits gnome-shell's **200000 µs**, which is what killed Ardour
+at 23:59. So if you always launch from the tray, **you were never exposed to that
+kill**; the icon path was. Do not go launch from the icon tonight to reproduce a
+bug you will not hit. On the tray path, the value of `fix-ardour-rt` is the *other*
+half: no GUI thread at FIFO/80 starving `AudioEngine` at 83 through RT bandwidth
+throttling (`sched_rt_runtime_us` 950000 = a 50 ms throttle, 2.3 quanta).
+
+Verified on the 11:05 instance: `AudioEngine 1` at FIFO/83, **nothing at 80**,
+graph back at 50 links / 39 SC ports, Master reaching the speakers.
+
+### 4. One loose end, for the sound-test hour and not before
+
+`midiUI`, `Generic MIDI` and `AutomationWatch` run at **SCHED_FIFO/92**, above
+`AudioEngine`'s 83. That is Ardour's own doing, not ours, and it is NOT to be
+touched tonight. But if xruns show up and they correlate with heavy LCXL3 knob
+movement rather than with the patterns, that is the first suspect.
+
+### Not a fault, so it does not go on a checklist
+
+`check-mix` FAIL and `fader-baseline` DRIFT compare the session against an
+**August 2 snapshot**. PLN rides the D-row up when he plays, so faders parked at
+`-inf` in a saved file are his workflow, not a bug — the 11:01 save simply
+recorded a parked desk. **The one number that is not explained by that is Master
+at −15.5 dB** (it was −0.5 at 00:14), because no MIDI binding for Master was found
+in the session (`<Binding>` count 0) or in `~/.config/ardour8/`. If Master is not
+under your hand, everything arrives 15.5 dB down. Ask before touching it.