feat(iso): split the live medium into semantic SquashFS layers

Booting via BMC virtual CD reads the ~2.8 GB filesystem squashfs
sequentially during the live-boot toram copy; a mid-read drop of the
redirected medium loses the whole copy and fails the boot (v14). Split
the rootfs into self-contained semantic layers so a retry re-reads at
most one ~500-700 MiB layer, not everything. This is a resilience /
reduced-re-read mechanism, not a fix for the virtual-media instability.

NVIDIA variants now ship 7 layers (00-base, 05-firmware, 08-desktop,
10-nvidia-driver, 20-nvidia-platform, 30-nvidia-cuda-libs,
40-nvidia-dcgm-cuda) plus an explicit live/filesystem.module that fixes
their OverlayFS order; amd/nogpu keep a single squashfs.

- lib/squashfs-layers.sh: deterministic classifier (dpkg file ownership
  plus explicit rules for build.sh-injected files, never a path
  substring), per-layer mksquashfs, 800 MiB hard ceiling, unsquashfs -s
  plus strict extraction of every layer, merged-rootfs bootability check.
- build.sh: split the monolith after the full lb build, verify and merge,
  write the module file, delete the monolith only then; abort before ISO
  assembly on any failure. Runs the builder test suites up front.
- fast-path: force a full build for a multi-layer medium;
  fast_path_repack_squashfs hard-refuses (it would drop layers).
- iso-validation.sh: validate_iso_squashfs_layers (module vs layer set
  match, size ceiling, no lone giant squashfs) and
  validate_iso_media_integrity (xorriso -check_media).
- bee-install: honour filesystem.module order, abort on any layer failure.
- 9013-toram-retry: record the real rsync exit code (it printed a false
  rc=0) and correct the "resumes the tail" comment (rsync without
  --partial keeps only fully-copied layers). No unsafe partial resume.
- tests: test-squashfs-layers.sh plus a multi-layer guard in
  test-build-libs.sh; both run at the top of every build.
- docs: bible-local architecture and decision, iso/README, iso-build-rules.

Verified by a full nvidia build: 7 layers 622/199/256/466/37/567/562 MiB,
every validator passes, xorriso -check_media good, merged rootfs bootable.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

bible-local/decisions/README.md

@@ -19,3 +19,4 @@ One file per decision, named `YYYY-MM-DD-short-topic.md`.
 | 2026-09-03 | PCIe link verdict comes only from the existing real-traffic GPU bandwidth SAT | active |
 | 2026-09-03 | Runtime Copy to RAM succeeds only after active loop devices move to tmpfs | active |
 | 2026-09-04 | Supported systems have at least 16 GB of RAM | active |
+| 2026-09-04 | Split the live medium into semantic SquashFS layers | active |