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>

iso/builder/test-build-libs.sh

@@ -155,5 +155,28 @@ if ! needs_full_build >/dev/null; then
     echo "ERROR: kernel ABI change did not force a full build" >&2
     exit 1
 fi
+printf '%s\n' "$DEBIAN_KERNEL_ABI" > "$FULL_BUILD_ABI_FILE"
+
+# A multi-layer live medium must always force a full build: the fast path only
+# repacks one squashfs and would silently drop the other layers.
+touch "$BUILD_WORK_DIR/binary/live/filesystem-v13.0-test-10-nvidia-driver.squashfs"
+if ! needs_full_build >/dev/null; then
+    echo "ERROR: a multi-layer squashfs medium did not force a full build" >&2
+    exit 1
+fi
+printf '%s\n' \
+    filesystem-v13.0-test-00-base.squashfs \
+    filesystem-v13.0-test-10-nvidia-driver.squashfs \
+    > "$BUILD_WORK_DIR/binary/live/filesystem.module"
+rm -f "$BUILD_WORK_DIR/binary/live/filesystem-v13.0-test-10-nvidia-driver.squashfs"
+if ! needs_full_build >/dev/null; then
+    echo "ERROR: filesystem.module alone did not force a full build" >&2
+    exit 1
+fi
+if ( fast_path_repack_squashfs ) >/dev/null 2>&1; then
+    echo "ERROR: fast_path_repack_squashfs ran on a multi-layer medium" >&2
+    exit 1
+fi
+rm -f "$BUILD_WORK_DIR/binary/live/filesystem.module"
 
 echo "build library tests: OK"