Add cooling.flow-under-load: the flow check with the tube lit through its window

A live catalog case: two 30 x 4 mm fills at full power on the press, the
window held open until the engine's flow verdict lands in the forgectrl
log, then M2 and the head brought back. PASS needs a verified line with
the laser's share on it (at least 0.3 C: the window and the fire
overlapped) and the judged rise at least 1 C under cool_flow_rise; an arm
refused by a gate names the gate. Passed on image 20260829190323
(heater rise 11.9 C judged, laser 0.6 off 12.5).

CAMPAIGN-LOG records the run and the day's two air-assist gate holds (a
bench connection, reseated); BRINGUP item 21 names the case.

Catalog: the case covers forgectrl src/cool.* and its neighbors; a new
test, so the manifest's coverage widens rather than moves.

docs/BRINGUP.md

@@ -1463,11 +1463,11 @@ Open items only. Anything closed is in `CAMPAIGN-LOG.md`.
     and toggles between two levels in between. Together they put an
     ordinary job's check within a few tenths of the limit. The engine now
     reads means and takes the tube's share off (`cool_laser_heat_cw`,
-    `cool_laser_heat_density`). Owed: a `cooling.*` catalog case with an
-    armed CW load; a re-measure of the two coefficients once a second
-    machine is on the bench (one tube, one supply so far); and if a lit
-    check still trips, the void-on-emission design with the tube as its
-    own flow tracer.
+    `cool_laser_heat_density`), and `cooling.flow-under-load` is the
+    catalog's case. Owed: a re-measure of the two coefficients once a
+    second machine is on the bench (one tube, one supply so far); and if a
+    lit check still trips, the void-on-emission design with the tube as
+    its own flow tracer.
     Open question, the offset's source: the timing points at the airflow
     drive (the step lands one sample after the fans go to run duty, before
     any HV, and lifts when they go idle, long after the tube is dark), but