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.

.github/workflows/yocto-cold-build.yml

@@ -30,10 +30,10 @@ jobs:
         with:
           path: forgefirm
 
-      # The kas config references meta-openglow as a local sibling, and
-      # meta-openglow's kernel-module bbappend uses an externalsrc sibling
-      # (both by design during active BSP development - kas/README.md
-      # "Push & release order" step 4 flips these at release time).
+      # The kas config references meta-openglow as a local sibling
+      # (kas/README.md "Push & release order" step 4 flips it to the
+      # pinned-remote block at release time). Every source repo the recipes
+      # build is fetched by pinned SRCREV; no other sibling is needed.
       - name: Checkout meta-openglow (sibling)
         uses: actions/checkout@v4
         with:
@@ -41,12 +41,6 @@ jobs:
           ref: scarthgap
           path: meta-openglow
 
-      - name: Checkout kernel-module-glowforge (sibling)
-        uses: actions/checkout@v4
-        with:
-          repository: ScottW514/kernel-module-glowforge
-          path: kernel-module-glowforge
-
       - name: Host dependencies
         run: |
           sudo apt-get update -qq