Fan floors from the measurement; a hunt is measured, not judged

forgectrl pin 47e4256: the airflow floors set from the bench measurement
(exhaust 6400, intake 2290, air assist 6000 rpm, purge current 300, grace
15 s) and the operating-point rule: a fan is judged while the laser is
armed, when a job's profile may raise it but never lower it below the run
duty, or whenever it is commanded at the run duty; the service's hunts,
sent with the extraction fans off, are measured and published unjudged.

Catalog: cloud.mode-switch now waits out the connect-time hunt sampling
/cool/status and requires no AIRFLOW, the exhaust row unjudged and the
exhaust actually off; its covers gain forgectrl src/cool.* and
src/airflow.*. cooling.fan-gate-trips uses an 8 s test grace (the
intakes take 7 s to 90 percent and tripped under the old 2 s at the new
floor) and its off leg waits for the row state as well as gates_off.
fan_floor_measure.py names a reply that is not JSON and calls the purge
readings idle and run (the pump is always on).

Docs: COOLING 3a and the settings table, BRINGUP item 19 and a facts-bank
entry with the measured fan speeds, the bench README, and a CAMPAIGN-LOG
entry for the measurement, the two status-document finds, the hunt find
and the hot-deployed bench runs of both tests.

scripts/bench/fan_floor_measure.py

@@ -10,7 +10,9 @@ purge-air current (sysfs head/purge_air_current, raw) once a second:
            reports the steady speed (the mean of the last --steady seconds),
            the time to 90 % of it, and the spread (min, max, standard
            deviation) over the steady window; for the purge fan the current
-           off and on. Needs the controller in GRBL mode, idle, fans quiet.
+           at idle and at run duty (the engine holds purge air on at all
+           times, so both are the "on" reading; a dead pump reads about 1).
+           Needs the controller in GRBL mode, idle, fans quiet.
 
   cut      No commands: samples for --seconds while the operator runs a
            real cut, and reports the spread the same way. This is the
@@ -56,7 +58,10 @@ def fans_now():
 
 def phase():
     st, body = forgectrl_request("GET", "/cool/status")
-    return body.get("phase") if st == 200 and isinstance(body, dict) else None
+    if st != 200 or not isinstance(body, dict):
+        raise SystemExit("GET /cool/status: HTTP %s with a body that is not a JSON object: %r"
+                         % (st, body if isinstance(body, str) else type(body).__name__))
+    return body.get("phase")
 
 
 def sample(seconds, label):
@@ -95,7 +100,7 @@ def spinup_report(rows, steady_s):
         t90 = next((r["t"] for r in rows if r[k] >= target), None) if target > 0 else None
         st["t90_s"] = t90
         out[k] = st
-    out["purge_on"] = stats([r["purge"] for r in steady])
+    out["purge_run"] = stats([r["purge"] for r in steady])
     return out
 
 
@@ -151,8 +156,8 @@ def main(argv):
             raise SystemExit("the engine is in phase %r; run this from idle" % ph)
         idle = fans_now()
         result["idle_fans"] = idle
-        result["purge_off"] = purge_current()
-        print("idle: fans %s purge %s" % (idle, result["purge_off"]))
+        result["purge_idle"] = purge_current()
+        print("idle: fans %s purge %s" % (idle, result["purge_idle"]))
         g = Grbl()
         try:
             print("M8:", g.cmd("M8"))
@@ -173,9 +178,9 @@ def main(argv):
             print("%-11s %8.0f %8d %8d %6.1f %7s" % (k, r.get("mean", 0), r.get("min", 0),
                                                    r.get("max", 0), r.get("sd", 0),
                                                    "%.0f s" % r["t90_s"] if r.get("t90_s") is not None else "-"))
-        po = rep["purge_on"]
-        print("purge current: off %s, on %.0f (min %s max %s)" % (result["purge_off"], po.get("mean", 0),
-                                                                 po.get("min"), po.get("max")))
+        po = rep["purge_run"]
+        print("purge current: idle %s, run %.0f (min %s max %s); the pump is always on, a dead one reads ~1"
+              % (result["purge_idle"], po.get("mean", 0), po.get("min"), po.get("max")))
         print()
         print("candidate floors at 55 %% of steady: " + "  ".join(
             "%s %.0f rpm" % (k, 0.55 * rep[k].get("mean", 0)) for k in FANS))