docs: present-state build and update docs; fix the ring-size arithmetic

- kas/README.md: the real-time rationale rests on the feeder's bounded
  queue depth, not ring size; the ring is 16 MiB (~84 s at 200 kHz,
  ~28 min at the 10 kHz cloud tick), a capacity for cloud-mode preload.
- BUILD.md, kas config, release checklist, cold-build workflow: only
  forgefirm and meta-openglow (branch scarthgap) are cloned as
  siblings; every ForgeFIRM source repo is fetched by pinned SRCREV.
- UPDATE-SYSTEM.md reads as the present-state design: the cloud-mode
  compatibility baseline is the cloud client's configured firmware
  version, not release metadata; decisions and open items listed
  plainly.
- README.md states what GRBL mode still needs the Glowforge service
  for (camera-referenced homing) and what runs without it.
- BRINGUP.md: generic build-host and fwup-lab references, the retained
  reproductions of the no-fire drill, the System tab.
- LIGHTBURN.md: the arm-window timeouts are machine settings.
- forgefirm-image.bb describes forgectrl as the machine-services
  daemon and points at the right backlog entry.
- American spelling throughout.

docs/LIGHTBURN.md

@@ -38,7 +38,9 @@ The laser fires only inside an operator-armed window:
   `M2`/`M30`), when the sender connection changes, or after
   `laser_disarm_s` (default 60 s) with the spindle off — counting even
   while a job sits paused in Hold or with the lid open; the next job
-  prompts again.
+  prompts again. Both timeouts are machine settings (keys in
+  `/data/forgefirm.conf`, set through the control panel's settings API);
+  the defaults suit normal use.
 - S-value scale: `$30` defaults to 1000, so set LightBurn's S-max to
   1000. 100 % power = S1000. Use M4 (variable/dynamic) mode for cuts
   and engraves.