usb_detect offered a Ventoy stick's 32M VTOYEFI ESP as a sync/backup
target: with the data partition unmounted it was the only mounted
candidate, and cp died mid-copy with 'No space left on device'.
Detection now reads FSTYPE/PARTTYPENAME and drops EFI system
partitions from both the mounted list and the mount-offer list;
USB_MOUNTED entries carry mp|label|size|model|fs and usb_pick_root
shows that in the single-stick confirm and the multi-stick/partition
picker (USB_ROOT stays a bare mountpoint). pos-media-sync pre-flights
the exact payload size vs df free space (err, or warn under --dry-run)
before any copy.