New crate: weft-mount-helper. A privileged helper binary that sets up
dm-verity devices and mounts EROFS images for app isolation.
Commands:
mount <img> <hash_dev> <root_hash> <mountpoint>
- opens a named dm-verity device via veritysetup open
- mounts the resulting /dev/mapper/<name> as EROFS read-only
- cleans up the dm device if mount fails
umount <mountpoint>
- unmounts the mountpoint
- closes the dm-verity device via veritysetup close
Device naming: derives a stable name from the mountpoint path, limited
to 31 chars (DM limit), always prefixed weft-. Root check reads
/proc/self/status euid rather than using unsafe libc calls.
Tests: device_name_sanitizes_path, device_name_truncates_long_paths.
