Pin forgectrl f9b4893: the air-assist ground shift off the coolant readings

The pin carries the compensation (cool_aa_offset_counts, the
aa-offset-calibrate diagnostic, the panel's Apply); verified with
bitbake -c fetch. The catalog gains cooling.aa-offset-calibrate, which
runs the tool and checks its recommendation and spread.

scripts/bench/offset_probe.py is the differential probe that found the
source: one actuator switched at a time with both sensors at 25 Hz
(survey), the air-assist duty ladder, the gantry jogging under the fan,
and an armed dark dwell. CAMPAIGN-LOG records the four probes and the
result; BRINGUP item 21 names the setting and the calibrate as what
remains on this machine, and item 23 opens the initial commissioning
procedure.

No catalog consequence beyond the new case: 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. |
+| `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. |
 | `flow_confirm_drill.py` | Coolant flow suspicion/confirmation drill (runs on the board): one continuous M8 session walks the verdict state machine through real pump-off transients — verified → SUSPECT (+ immediate re-check) → cleared → SUSPECT → FAULT (consecutive) → recovered — printing PASS/FAIL per transition. Leaves the machine idle (M9, pump on, heater off). |