Bench: fan_floor_measure.py, the numbers the airflow gates ship with

Two modes. spinup opens a run session with M8 from the engine's idle
posture, samples the four tachometers and the purge-air current once a
second, ends the session with M9 and waits for the engine to leave run;
per fan it reports the steady speed at run duty, the time to 90 percent
of it and the spread over the steady window, the purge current off and
on, and candidate floors at 55 percent of steady. cut sends nothing and
samples while the operator runs a real cut, for the spread the debounce
has to ride over under the only load that matters. Registered on the
bench page as fan-floor. Tooling only; no catalog consequence.

forgetest/forgetest/bench.py

@@ -152,6 +152,15 @@ TOOLS = [
      "safety": "dry", "where": "board", "ported": True, "args": [],
      "desc": "Snapshots fan PWMs/tachs/temps, drives M8 -> cut fans, M9 -> cooldown -> idle; the tach "
              "readbacks in each snapshot are the evidence."},
+    {"id": "fan-floor", "title": "Fan floor measurement", "script": "fan_floor_measure.py",
+     "safety": "dry", "where": "board", "ported": True,
+     "args": [_arg("mode", "choice", "spinup", "spinup (M8, from idle) / cut (sample only, during a real cut)",
+                   ["spinup", "cut"]),
+              _arg("seconds", "int", 120, "sampling window", flag="--seconds"),
+              _arg("steady", "int", 60, "spinup: the steady window at the end", flag="--steady")],
+     "desc": "The numbers the airflow gates ship with: per fan the steady speed at run duty, the time to "
+             "90 percent and the spread, the purge current off and on, and (cut) the spread under a real "
+             "cut; results to the bench data directory."},
     # -- laser (live) --------------------------------------------------------------
     {"id": "live-fire", "title": "LIVE laser drills", "script": "live_fire_drills.py",
      "safety": "live", "where": "board", "ported": True,