fix: restore -partition_offset 16 to xorriso for USB boot compatibility

The old Debian Live ISO used -partition_offset 16 which reserves space
for a GPT partition table in the hybrid MBR layout. UEFI firmware on
some machines requires this to recognize the USB as bootable. We
removed it thinking it was Debian Live-specific but it's actually an
xorriso hybrid boot requirement.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

image-recipe/build-auto-installer-iso.sh

@@ -2602,6 +2602,7 @@ if [ ! -f "$EFI_IMG" ]; then
         -c isolinux/boot.cat \
         -b isolinux/isolinux.bin \
         -no-emul-boot -boot-load-size 4 -boot-info-table \
+        -partition_offset 16 \
         "$INSTALLER_ISO"
 else
     xorriso -as mkisofs -o "$OUTPUT_ISO" \
@@ -2616,6 +2617,7 @@ else
         -e boot/grub/efi.img \
         -no-emul-boot \
         -isohybrid-gpt-basdat \
+        -partition_offset 16 \
         "$INSTALLER_ISO"
 fi