Pin forgectrl 42cdb71; add the coolant offset check; record the compensation on the bench

The pin carries the calibrate tool's sampling fix (3 s at 8 Hz a side).
aa_offset_check.py proves the correction on a machine: M8 brings the fans
to the run profile, dark, while the raw coolant counts, /status and the
engine's readings are averaged before, during and after; it turns the
flow check off for its session and restores it. On the bench page as
aa-offset-check. CAMPAIGN-LOG records the tool's first run, the applied
value (16 counts) and the proof: the readings held within 0.1 C under
the run profile where they dropped 1.0 C uncorrected. BRINGUP item 21
names what remains.

No catalog consequence: a bench tool and records.

scripts/bench/README.md

@@ -42,6 +42,7 @@ page's takeover does that; from a host, stop them first.
 | `flow_sustained.py` | Long-run test of the real re-check cadence via M8 (board or host; controller running): counts verdicts/false faults against the configured `cool_flow_rise` and tracks whether the loop accumulates heat. `flow_sustained.py [minutes]`. |
 | `temp_calibrate.py` (`supply-*` modes) | The power supply's sensor (`pic/pwr_temp`, raw) against a thermometer on its heatsink: `supply-watch`, `supply-point <C>`, `supply-fit`; the fit is printed beside `UAPI.md`'s unverified guess. Three points during a long cut settle it. |
 | `critical_tier_drill.py` | The coolant critical tier on a rising temperature (board or host): sets the ceiling, the resume gate and the critical line a few tenths above the live upstream reading and lets the engine's own flow-check heater warm the loop through them inside one `M8` session, expecting `OVERTEMP` at the ceiling and then `CRITICAL` (fire blocked, hold, no resume) with the fault ending at `M9`; restores the settings and cycles a session so the engine re-reads them. Results as JSON in the bench data directory. |
+| `aa_offset_check.py` | Coolant offset correction under the run airflow (board; controller running; dark, no press): M8 brings the fans to the run profile while the raw coolant counts, `/status` and the engine's readings are averaged before, during and after; with `cool_aa_offset_counts` at the machine's value the readings hold still while the raw counts step, at zero they drop by about a degree. `aa_offset_check.py [dwell_s]`. |
 | `offset_probe.py` | Coolant-sensor offset probe (board; forgectrl idle; dark, no press): switches one actuator at a time (exhaust at 100/50/25 %, intakes, air assist, purge, heater, pump, TEC, lid lamp, then all run fans) with both thermistors sampled at 25 Hz and scores the common-mode step at every edge and the level toggling inside every dwell; `offset_probe.py ladder` runs the air-assist duty ladder alone. Every value is restored on exit. JSON record in the bench data directory. |
 | `flow_warm_validate.py` | Runs the real check from a heater-warmed baseline (board or host; forgectrl and controller stopped; `flow_warm_validate.py [cycles_per_case]`; results/log in the bench data directory; exit 1 if any run is misclassified). Note the ceiling: 100 % duty pushes the downstream sensor past 50 °C in 30 s while the bulk barely moves, so warm-loop validation above ~23 °C needs the laser, not the heater. |
 | `flow_recheck_char.py` | Characterizes short in-run re-checks and the differential metric (board or host; forgectrl and controller stopped; `flow_recheck_char.py [heater_pct] [window_s]`); shows why over-temp cannot see a stopped pump and why passive warming trends are ambiguous. |