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.

meta-forgefirm/recipes-forgefirm/images/forgefirm-image.bb

@@ -5,13 +5,14 @@ DESCRIPTION = "OpenGlow/ForgeFIRM image for Glowforge"
 # ForgeFIRM cuts the cloud dependency: drop the Glowforge cloud client
 # (gfui-client connects to Glowforge's servers). Removing it here (override
 # only; the shared glowforge-image base is untouched) also sidesteps its
-# do_package failure. Its slot is filled locally by forgectrl (camera MJPEG
-# service; the grblHAL controller recipe is tracked in kas/README.md
-# backlog #5).
+# do_package failure. Its role is filled locally by forgectrl and the
+# controllers below (kas/README.md backlog #6).
 IMAGE_INSTALL:remove = "gfui-client"
 
 # grblhal-glowforge: the grblHAL motion controller (Grbl over TCP:23).
-# forgectrl: the ForgeFIRM control daemon (camera MJPEG service on :8080).
+# forgectrl: the ForgeFIRM machine-services daemon (HTTP :8080): controller
+# supervisor, pulse-device broker, cooling engine, cameras, telemetry,
+# settings, diagnostics, web control panel, and A/B updates.
 # gfhome: one-shot Glowforge web-service homing, invoked by the controller
 # for $H when homing_mode = gfcloud (/data/forgefirm.conf).
 # gfcloud: full Glowforge web-service controller daemon (the factory cloud