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_sampler.py

@@ -0,0 +1,22 @@
+#!/usr/bin/env python3
+"""Board-side coolant sampler: prints 'elapsed,raw_down,raw_up' lines.
+Usage: flow_sampler.py <duration_s> <interval_s>
+Kept on the board so sampling cadence is not at the mercy of ssh
+round-trip latency."""
+import sys
+import time
+
+dur = float(sys.argv[1])
+iv = float(sys.argv[2])
+t0 = time.time()
+
+while True:
+    el = time.time() - t0
+    if el > dur:
+        break
+    with open('/sys/glowforge/pic/water_temp_1') as f:
+        r1 = f.read().strip()
+    with open('/sys/glowforge/pic/water_temp_2') as f:
+        r2 = f.read().strip()
+    print('%.2f,%s,%s' % (el, r1, r2), flush=True)
+    time.sleep(iv)