fix(iso): stop fast-path repack from clobbering merged-usr /lib

The NVIDIA GSP firmware was staged into the overlay at a bare
lib/firmware/nvidia/<ver>/. On a merged-usr Debian root /lib is a symlink
to usr/lib, so the fast-path repack's `rsync -a` (overlay stage -> the
unsquashfs'd tree) replaced that symlink with a plain directory holding
only the firmware. Everything else reachable only through /lib
(/lib/systemd/systemd, /lib/x86_64-linux-gnu/ld-linux + libc, ...) was
orphaned. The squashfs still packed and mounted, but boot panicked:

    run-init: can't execute '/sbin/init': No such file or directory
    run-init: can't execute '/bin/sh': No such file or directory
    Kernel panic - not syncing: Attempted to kill init

Full `lb build` was unaffected (live-build copies includes.chroot with
cp -a, which follows the /lib symlink), so only fast-path ISOs were bad.

Three fixes:
- build.sh: stage GSP firmware under usr/lib/firmware/nvidia/<ver>/, the
  canonical merged-usr path. Changing the overlay path also makes
  overlay_paths_were_removed() force one full build on the next run.
- fast-path.sh: add `rsync --keep-dirlinks` so a real directory in the
  overlay stage can never again replace a symlink-to-directory in the
  root. Verified: firmware lands in usr/lib/firmware, /lib stays a symlink.
- iso-validation.sh: new validate_iso_rootfs_layout, run for every variant
  on both build paths. Fails the build if the squashfs has a plain-dir
  /bin|/sbin|/lib|/lib64, or if /usr/sbin/init is present without a
  resolvable /usr/lib/systemd/systemd and /lib symlink. Verified it flags
  the broken v14.02-1-g642e686 ISO and passes a correct layout.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01H2LLuid8PFhBqBPcxXxkQU

iso/builder/lib/fast-path.sh

@@ -135,7 +135,12 @@ fast_path_repack_squashfs() (
     echo "=== fast-path: unsquash $(basename "$_old_sq") ($(du -sh "$_old_sq" | cut -f1) compressed) ==="
     unsquashfs -d "$_tmp" "$_old_sq"
     echo "=== fast-path: syncing overlay stage ==="
-    rsync -a --checksum "${OVERLAY_STAGE_DIR}/" "$_tmp/"
+    # --keep-dirlinks: when the extracted root has a symlink-to-directory (e.g.
+    # merged-usr /lib -> usr/lib) and the overlay stage carries a real directory
+    # of the same name, follow the symlink instead of replacing it with a plain
+    # directory. Without this rsync silently orphans everything reachable only
+    # through that symlink and the resulting image panics at boot.
+    rsync -a --keep-dirlinks --checksum "${OVERLAY_STAGE_DIR}/" "$_tmp/"
     echo "=== fast-path: repacking as ${SQUASHFS_FILENAME} ==="
     _sq_new="${_sq}.new"
     rm -f "$_sq_new"

iso/builder/lib/iso-validation.sh

@@ -639,3 +639,64 @@ validate_iso_nvidia_runtime() {
     rm -rf "$dpkg_status_dir"
     echo "=== NVIDIA runtime validation OK ==="
 }
+
+rootfs_layout_fail() {
+    echo "ERROR: $1" >&2
+    exit 1
+}
+
+# Guard against a corrupted merged-usr layout in the live rootfs. On Debian
+# bookworm /bin, /sbin, /lib and /lib64 are symlinks into /usr; if a build step
+# stages a real directory of that name into the overlay, `rsync -a` replaces the
+# symlink with a plain directory and orphans everything reachable only through
+# it (/lib/systemd/systemd, /lib/x86_64-linux-gnu/*, ...). The image still packs
+# and mounts fine but panics at boot: "run-init: can't execute '/sbin/init'".
+validate_iso_rootfs_layout() {
+    iso_path="$1"
+    echo "=== validating rootfs merged-usr layout in ISO ==="
+
+    [ -f "$iso_path" ] || rootfs_layout_fail "ISO not found for rootfs layout validation: $iso_path"
+    require_iso_reader "$iso_path" >/dev/null 2>&1 || rootfs_layout_fail "ISO reader unavailable for rootfs layout validation"
+    command -v unsquashfs >/dev/null 2>&1 || rootfs_layout_fail "unsquashfs is required for rootfs layout validation"
+
+    _sq_tmp="$(mktemp)"
+    _sq_list="$(mktemp)"
+    _iso_files="$(mktemp)"
+
+    iso_list_files "$iso_path" > "$_iso_files" \
+        || rootfs_layout_fail "failed to list ISO files for rootfs layout validation"
+
+    _saw_rootfs=0
+    while IFS= read -r _member; do
+        [ -n "$_member" ] || continue
+        iso_read_member "$iso_path" "$_member" "$_sq_tmp" \
+            || rootfs_layout_fail "failed to extract $_member from ISO"
+        : > "$_sq_list"
+        unsquashfs -ll "$_sq_tmp" > "$_sq_list" 2>/dev/null \
+            || rootfs_layout_fail "failed to inspect $_member from ISO"
+
+        # A plain-directory top-level bin/sbin/lib/lib64 is always wrong here.
+        _bad="$(grep -E '^d[rwxsStT-]{9} .* squashfs-root/(bin|sbin|lib|lib64)$' "$_sq_list" || true)"
+        if [ -n "$_bad" ]; then
+            echo "$_bad" >&2
+            rootfs_layout_fail "$_member: /bin|/sbin|/lib|/lib64 is a plain directory, not a merged-usr symlink (overlay clobbered it; stage under usr/lib instead)"
+        fi
+
+        # In the layer that carries the base rootfs, assert init is resolvable.
+        if grep -Eq ' squashfs-root/usr/sbin/init( ->|$)' "$_sq_list"; then
+            _saw_rootfs=1
+            grep -Eq ' squashfs-root/usr/lib/systemd/systemd$' "$_sq_list" \
+                || rootfs_layout_fail "$_member: /usr/sbin/init present but /usr/lib/systemd/systemd missing"
+            grep -Eq ' squashfs-root/lib -> ' "$_sq_list" \
+                || rootfs_layout_fail "$_member: /lib is not a symlink into /usr (merged-usr broken)"
+        fi
+    done <<EOF
+$(grep -E '^live/.*\.squashfs$' "$_iso_files")
+EOF
+
+    [ "$_saw_rootfs" = 1 ] \
+        || rootfs_layout_fail "no squashfs layer with /usr/sbin/init found in ISO"
+
+    rm -f "$_sq_tmp" "$_sq_list" "$_iso_files"
+    echo "=== rootfs layout validation OK ==="
+}