Add laser-safety and regulatory documentation; scrub bench identity

- LIGHTBURN.md: mandatory "Before you cut" safety section; the
  walkthrough now reflects the firing machine (dry runs need the
  layer output off or M5; live first-cut instructions); the homing
  entry documents homing_mode and the gfcloud method; the machine
  address is a placeholder.
- README.md: condensed safety section linking the full text and the
  regulatory notes.
- INSTALL.md: "Regulatory and legal" section ahead of the install
  steps; routine updates route through the panel updater rather than
  the installer.
- BRINGUP.md: the release signing key is described as held offline
  (no on-disk path); bench address and credential notes removed;
  Next-work item 7 corrected (the installer embeds the production
  release key); status entry for audit remediation Phases 0-1; the
  GATE A kernel drills join the pending image-flash checklist.
- bench scripts: the target host comes from GF_HOST (or argv) instead
  of a hardcoded address.
- laser_stream_test.py: per-session controller runs with a hermetic
  cooling-verdict publisher; new assertions that every stream
  terminates with FIRE clear (including M3 held to stream end) and
  that no FIRE bit rides a zero-step gap; a cycle-churn session
  exercises the stop/start seams.

Audit findings D-1, D-2, D-3, D-5, D-10, D-12, B-10, and the harness
half of D-4/G-1.

scripts/bench/bench_m2.py

@@ -2,7 +2,7 @@
 """Milestone-2 motion-quality bench: factory-true rates/accels over TCP.
 
 Runs a bounded, return-to-start jog sequence against grblHAL on the board
-(default 172.16.1.97:23) and reports peak feed reached, state transitions,
+(argv[1] or GF_HOST, port 23) and reports peak feed reached, state transitions,
 and final position drift. Every move is relative and round-trip, so the
 head ends where it started; the laser stays latched (motion-only backend).
 
@@ -10,11 +10,14 @@ Sequence: sanity jogs (X, Y, 40 mm out/back at 2400 mm/min), max-rate X
 out/back (60 mm at F12000 - peaks ~200 mm/s mid-move), diagonal out/back,
 then a G1 move with a feed-hold/resume in the middle.
 """
+import os
 import socket
 import sys
 import time
 
-HOST = sys.argv[1] if len(sys.argv) > 1 else '172.16.1.97'
+HOST = sys.argv[1] if len(sys.argv) > 1 else os.environ.get('GF_HOST')
+if not HOST:
+    raise SystemExit('pass the machine IP as argv[1] or set GF_HOST')
 PORT = 23