scripts: guard enable-hibernation against empty root PARTUUID

If `findmnt -no PARTUUID /` returns nothing (root on LVM, no GPT,
unusual mount state) the script would silently write a broken
resume=PARTUUID= line to /etc/kernel/cmdline. Bail with an error
message instead.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

scripts/enable-hibernation.sh

@@ -13,6 +13,7 @@ fi
 SWAPFILE=/swapfile
 SWAPSIZE=16g
 ROOT_PARTUUID=$(findmnt -no PARTUUID /)
+[ -n "$ROOT_PARTUUID" ] || { echo "ERROR: could not determine root PARTUUID via findmnt." >&2; exit 1; }
 
 echo "==> Step 1/6: ensure swap file exists at $SWAPFILE (size $SWAPSIZE)"
 if [ ! -f "$SWAPFILE" ]; then