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.

README.md

@@ -55,6 +55,17 @@ The control board is common to Glowforge Basic, Plus, and Pro. The 5 MP
 * Cloud mode: stream jobs into the motion ring during the run, lifting the
   job-length cap that buffering the whole job imposes.
 
+## Safety
+
+**This machine contains a Class 4 CO₂ laser: it burns, blinds, and starts
+fires.** Never defeat the lid switches or interlock, always vent the exhaust
+outdoors, and never cut PVC or other chlorinated plastics. Never leave a
+running job unattended — keep a fire extinguisher within reach. Read
+[Before you cut — safety](docs/LIGHTBURN.md#before-you-cut--safety-read-this-first)
+before your first job, and the
+[Regulatory and legal](INSTALL.md#regulatory-and-legal) section before
+installing.
+
 **A very important warning: this is experimental software. Use of this software
 could seriously maim or kill you or others, and voids your warranty. It is not
 affiliated with or endorsed by Glowforge. Use it at your own risk.**