Flow-detection experimental record and bench tooling

Adds the design matrix and its supporting tools, and records in BRINGUP
what the 60-run matrix overturned: sub-40-percent duty mimics flow
(three of five dead-pump trials looked healthier than a working pump),
the operating point and threshold now rest on 25 pooled observations,
periodic re-checks are thermally free with the fans running, and the
settle gate closes a bench-proven miss. Also records what is NOT yet
validated - warm-loop baselines and behaviour under laser heating - as
first-light commissioning items.

scripts/bench/flow_recheck_char.py

@@ -0,0 +1,110 @@
+#!/usr/bin/env python3
+"""Characterize a SHORT periodic flow re-check for use DURING a job.
+
+Why a separate design from the job-start check: with the laser firing
+and coolant flowing, tube heat raises BOTH sensors, inflating a plain
+downstream-rise metric toward a false fault. With the pump stopped that
+heat never reaches the sensors at all (stagnant loop), so the no-flow
+signature stays at its idle value. The two signatures therefore
+converge during a cut. The differential rise
+
+    (downstream_end - downstream_start) - (upstream_end - upstream_start)
+
+cancels that common-mode heating and is the metric this measures.
+
+TWO DEAD ENDS, recorded so they are not re-invented:
+
+1. Absolute over-temperature does NOT detect a failed pump. The
+   sensors read the water at their own location; with no circulation
+   the tube's heat stays in the tube and never reaches them. The loop
+   can read perfectly comfortable while the tube cooks. Over-temp
+   monitoring detects a hot CIRCULATING loop - a different failure.
+
+2. "The coolant should warm up while cutting" is NOT a usable
+   indicator either. A low-duty engrave (say 5% duty at 30% power) can
+   put so little heat in that the cooling system absorbs it with no
+   measurable rise, so a flat trend is equally consistent with a light
+   load working correctly and with a dead pump. Ambiguous evidence is
+   worse than none - it invites false confidence.
+
+Hence: periodic ACTIVE interrogation with the heater, which creates a
+known stimulus instead of waiting for one, is the only valid method on
+this hardware (there is no pump tach or pump current sense anywhere in
+the machine).
+
+Usage: flow_recheck_char.py [heater_pct] [window_s]   (default 50 30)
+Runs both flow and no-flow cases from a comparable loop state and
+prints the differential separation. Aborts if downstream passes 45 C.
+"""
+import math
+import subprocess
+import sys
+import time
+
+HOST = '172.16.1.97'
+F = 1024.0 * 1.3
+RD, BETA = 10000.0, 3380.0
+RINF = 10000.0 * math.exp(-3380.0 / 298.15)
+DOWN_ABORT_C = 45.0
+
+
+def degc(raw):
+    r = RD / (F / raw - 1.0)
+    return BETA / math.log(r / RINF) - 273.15
+
+
+def board(cmd):
+    r = subprocess.run(['wsl', '-d', 'forge-yocto', '--', 'ssh',
+                        '-o', 'PreferredAuthentications=none',
+                        'root@' + HOST, cmd],
+                       capture_output=True, text=True, timeout=30)
+    return r.stdout.strip()
+
+
+def sample():
+    o = board('cat /sys/glowforge/pic/water_temp_1 /sys/glowforge/pic/water_temp_2').split()
+    return degc(int(o[0])), degc(int(o[1]))
+
+
+def settle(seconds=100):
+    board('echo 1 > /sys/glowforge/thermal/water_pump_on; echo 0 > /sys/glowforge/thermal/heater_pwm')
+    time.sleep(seconds)
+
+
+def check(tag, pump_on, pct, window):
+    board('echo %d > /sys/glowforge/thermal/water_pump_on' % (1 if pump_on else 0))
+    time.sleep(2)
+    d0, u0 = sample()
+    board('echo %d > /sys/glowforge/thermal/heater_pwm' % int(65535 * pct / 100))
+    t0 = time.time()
+    d, u = d0, u0
+    while time.time() - t0 < window:
+        time.sleep(5)
+        d, u = sample()
+        print('    %-8s t=%2.0fs  down=%5.2f (%+5.2f)  up=%5.2f (%+5.2f)  diff=%+5.2f'
+              % (tag, time.time() - t0, d, d - d0, u, u - u0, (d - d0) - (u - u0)), flush=True)
+        if d >= DOWN_ABORT_C:
+            print('    abort: downstream at safety limit', flush=True)
+            break
+    board('echo 0 > /sys/glowforge/thermal/heater_pwm; echo 1 > /sys/glowforge/thermal/water_pump_on')
+    return (d - d0) - (u - u0), d - d0
+
+
+pct = int(sys.argv[1]) if len(sys.argv) > 1 else 50
+window = int(sys.argv[2]) if len(sys.argv) > 2 else 30
+
+print('=== periodic re-check characterization: %d%% heater, %d s window' % (pct, window))
+settle()
+print('  flow case:')
+flow_diff, flow_rise = check('flow', True, pct, window)
+settle()
+print('  no-flow case:')
+noflow_diff, noflow_rise = check('noflow', False, pct, window)
+settle(60)
+
+print()
+print('differential rise:  flow %+.2f   no-flow %+.2f   separation %.2f C'
+      % (flow_diff, noflow_diff, noflow_diff - flow_diff))
+print('plain down rise:    flow %+.2f   no-flow %+.2f   separation %.2f C'
+      % (flow_rise, noflow_rise, noflow_rise - flow_rise))
+print('suggested differential threshold: %.2f C' % ((flow_diff + noflow_diff) / 2))