Stage 3: the CSS bus-authentication (the challenge-response that sets the
drive's ASF=1 to unlock scrambled-sector reads, plus crypt_key + the CSS
tables) moves out of libfreemkv into the self-contained src/css module, with
its own error type. CssUnlocker impls crate::Unlocker — matches DiscKind::Css,
self-guards against a non-DVD profile, runs the bus-auth, and returns an empty
Unlocked (CSS yields no VID/bus key; the keyless Stevenson descramble stays in
libfreemkv). The keyless descramble (lfsr/stevenson) is NOT moved — that's
content decryption, the consumer's job.
all_unlockers() now returns [LibreDrive, AacsCert, CssUnlocker] — the entire
unlock layer is built. libfreemkv rewires onto it in stage 4. 89 tests pass.
