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.

README.md

@@ -24,7 +24,9 @@ machine is idle:**
   for each job.
 * **Cloud mode** — the machine presents itself as a stock Glowforge to the
   Glowforge web service, so the phone and web apps work as they always did.
-  Optional, and off by default; nothing about GRBL mode needs it.
+  Optional, and off by default. GRBL mode jogs and cuts without it; the one
+  GRBL-mode function that still reaches the Glowforge service is
+  camera-referenced homing (below), until limit-switch homing lands.
 
 **Around both modes:**
 
@@ -34,8 +36,9 @@ machine is idle:**
 * Both **cameras** as MJPEG streams and full-resolution snapshots — the lid
   camera feeds LightBurn's camera overlay directly.
 * **Camera-referenced homing**: `$H` from any sender runs the factory-style
-  camera homing cycle through the Glowforge service, and the machine records
-  where it is.
+  camera homing cycle through the Glowforge service (a Glowforge account and a
+  live service session are required for `$H`; everything else in GRBL mode
+  runs without them), and the machine records where it is.
 * **Installs alongside the factory firmware** in the unused A/B rootfs slot,
   archiving every factory version first, so the machine can be switched back
   to stock at any time without the Glowforge cloud.