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.

kas/forgefirm-glowforge.yml

@@ -73,11 +73,10 @@ repos:
       .:
 
   # --- meta-openglow — Glowforge BSP layers ---------------------------------
-  # ACTIVE DEVELOPMENT: referenced as a local sibling checkout (no url => kas
-  # performs no git ops, so the scarthgap-migration edits we're making are what
-  # gets built). The kernel-module-glowforge sources are NOT a layer; they are
-  # pulled by the kernel-module-glowforge.bb recipe's SRC_URI, so kas does not
-  # manage them here.
+  # Referenced as a local sibling checkout (no url => kas performs no git ops,
+  # so in-place BSP edits are what gets built). The kernel-module-glowforge
+  # sources are NOT a layer; the kernel-module-glowforge.bb recipe fetches them
+  # by pinned SRCREV, so kas does not manage them here.
   meta-openglow:
     path: ../meta-openglow
     layers:
@@ -85,11 +84,8 @@ repos:
       meta-glowforge-bsp:
       # meta-openglow-bsp (separate OpenGlow_std board) intentionally excluded.
   #
-  # FUTURE — flip to this pinned-remote block at release time (gated on
-  # active BSP development settling — see kas/README.md "Push & release order".
-  # When flipping, also drop meta-openglow's kernel-module-glowforge.bbappend
-  # (externalsrc to the local sibling) so a fresh clone is self-contained; its
-  # perl-native DEPENDS is already carried in the base recipe):
+  # Pinned-remote alternative for a fully self-contained clone (see
+  # kas/README.md "Push & release order"):
   #   meta-openglow:
   #     url: https://github.com/ScottW514/meta-openglow.git
   #     branch: scarthgap          # pin via kas lock / a tag at release
@@ -122,9 +118,7 @@ local_conf_header:
   # release.sh gates the release rootfs against a passwordless root entry.
 
   build-tweaks: |
-    # Parallelism for the 12-core / 16 GB WSL2 VM. (The earlier mid-build deaths
-    # were the mirrored-networking vsock relay, not memory — see .wslconfig — so
-    # this can comfortably use more of the VM.)
+    # Parallelism sized for a 12-core / 16 GB build VM; raise on larger hosts.
     BB_NUMBER_THREADS = "8"
     PARALLEL_MAKE     = "-j 8"
     # Caches kept inside the forgefirm repo (gitignored):