Fan/thermal control done and bench-verified; docs updated

BRINGUP records the gate as complete with tach-verified results;
LIGHTBURN.md documents the per-layer Air Assist toggle driving the
cut-profile ventilation; fan_test.py joins the bench kit.

scripts/bench/fan_test.py

@@ -0,0 +1,59 @@
+import socket, subprocess, time
+
+HOST = '172.16.1.97'
+
+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().replace('\n', ' ')
+
+ATTRS = ('head/air_assist_pwm head/air_assist_tach head/purge_air '
+         'thermal/exhaust_pwm thermal/tach_exhaust '
+         'thermal/intake_pwm thermal/tach_intake_1 thermal/tach_intake_2 '
+         'thermal/water_pump_on thermal/tec_on pic/water_temp_1 pic/water_temp_2')
+
+def snap(tag):
+    vals = board('cd /sys/glowforge && cat ' + ATTRS).split()
+    keys = [a.split('/')[-1] for a in ATTRS.split()]
+    print(tag)
+    for k, v in zip(keys, vals):
+        print('  %-18s %s' % (k, v))
+
+s = socket.create_connection((HOST, 23), timeout=5)
+s.settimeout(0.15)
+def drain():
+    out = b''
+    try:
+        while True:
+            d = s.recv(4096)
+            if not d: break
+            out += d
+    except socket.timeout: pass
+    return out.decode('ascii', 'replace')
+def cmd(l, w=0.4):
+    s.sendall(l.encode() + b'\n'); time.sleep(w); return drain().strip()
+
+time.sleep(0.5); drain()
+
+snap('--- baseline (driver idle profile)')
+w2 = board('cat /sys/glowforge/pic/water_temp_2')
+print('water_temp_2 = %s -> %.1f C' % (w2, int(w2) * -0.09653 + 94))
+
+print()
+print('=== M8: cut-profile fans ON (loud) ===')
+print('M8:', cmd('M8'))
+time.sleep(4)                      # spin-up
+snap('--- during M8')
+
+print()
+print('=== M9: cooldown (fans stay on ~15 s) ===')
+print('M9:', cmd('M9'))
+time.sleep(3)
+snap('--- during cooldown (should still be at run values)')
+time.sleep(14)                     # past the 15 s cooldown
+snap('--- after cooldown (should be idle: AA 204, EF 0, IF 0)')
+s.sendall(b'?'); time.sleep(0.3)
+t = drain()
+print('grbl:', t[t.rfind('<'):t.rfind('>') + 1] if '<' in t else t)