Expose is_unlock_read_buffer() and UNLOCK_MARKER so consumers (bdemu)
can recognise and answer the LibreDrive unlock READ_BUFFER handshake
without open-coding the variant CDB shapes or the verification marker.
These handshake internals must live only in freemkv-unlock-ld.
libfreemkv owns the Unlocker trait + registry; this repo holds the concrete
unlocker plugins. LibreDrive moves to the ld/ member (package freemkv-unlock-ld
unchanged). Workspace root + repo README describe the general-unlocker model;
adding an unlocker = a new member + one register_unlocker line in the consumer.
libfreemkv dep is the crates.io version; local dev resolves it via the
gitignored .cargo/config.toml patch.
Matthew Jackson