Recover the per-drive READ_BUFFER OEM VID path that lived in libfreemkv
before the unlocker refactor (read_vid_oem in disc/encrypt.rs), now living
inside the unlocker where the per-drive read_vid_cdb template belongs.
read_vid finds the drive's profile; if it carries a read_vid_cdb, issues
it and parses the 36-byte response (signature 00 22 00, VID at [4..20]),
returning Some(vid). No matching profile or no OEM VID CDB returns Ok(None)
so libfreemkv falls back to the cert handshake. VID is thus obtained
without the host certificate + HRL.
Adds response-parse unit tests (well-formed, short response, bad header,
no-profile fallback).
Matthew Jackson
d6d79fc74c