installer-iso: persist auto-install logs to the staging disk

scripts/deploy

@@ -342,6 +342,17 @@ local_install_prepare_and_reboot() {
   [ "$stage_fstype" != btrfs ] \
     || die "$stagedir is btrfs: findiso= mounts the volume's top level, so a path inside a subvolume never resolves. Stage on a non-btrfs partition (ext4/vfat/ntfs)."
 
+  # PARTUUID of the staging partition. Handed to the installer as
+  # homelab.logpart= so it can mount this partition rw and persist its whole
+  # run — disko + nixos-install output included — to a file next to the iso.
+  # This partition is on a DIFFERENT disk from the one disko wipes (guarded
+  # above), so unlike $boot it SURVIVES the install: a failed attempt otherwise
+  # leaves nothing to debug, its journal having died on tmpfs at the reboot.
+  # Best-effort — an LVM/mdraid stage_src has no PARTUUID, in which case logging
+  # is simply skipped rather than blocking the install.
+  local stage_partuuid
+  stage_partuuid="$(lsblk -no PARTUUID "$stage_src" 2>/dev/null | head -1 | tr -d ' ' || true)"
+
   # Last chance to back out. This is the most destructive command in the
   # script — it reboots the machine you are typing at and the wipe that
   # follows is unattended — so it confirms just like `flash` and `kexec-local`
@@ -354,6 +365,11 @@ local_install_prepare_and_reboot() {
     echo "              -> $osdisk_real  ** WIPED, unattended, after the reboot **"
     # Unquoted on purpose: collapses the one-per-line list onto one line.
     echo "   staging:   $stagedir  (on $(echo $stage_disks))"
+    if [ -n "$stage_partuuid" ]; then
+      echo "   logs:      $stagedir/homelab-install-$config.log  (on the staging disk — survives the wipe)"
+    else
+      echo "   logs:      (none — $stagedir has no PARTUUID; installer output won't survive the wipe)"
+    fi
     echo "   one-shot:  $boot_mode"
     read -rp ">> type 'yes' to build the installer, reboot into it and wipe $osdisk_real: " ok
     [ "$ok" = yes ] || die "aborted"
@@ -426,6 +442,9 @@ local_install_prepare_and_reboot() {
   local cmdline volumeID
   volumeID="$(nix eval --raw .#nixosConfigurations.installer-iso.config.isoImage.volumeID)"
   cmdline="init=$toplevel/init nohibernate root=LABEL=$volumeID boot.shell_on_fail loglevel=4 lsm=landlock,yama,bpf findiso=$iso_relpath homelab.install=$config homelab.keypart=$boot_partuuid"
+  # Only when the staging partition has a PARTUUID (see stage_partuuid). Points
+  # the installer's homelab-auto-install.service at the surviving disk to log to.
+  [ -n "$stage_partuuid" ] && cmdline="$cmdline homelab.logpart=$stage_partuuid"
 
   case "$boot_mode" in
     systemd-boot)