css: relocate the CSS bus-auth into freemkv-unlock; all 3 unlockers wired

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.

src/lib.rs

@@ -13,8 +13,8 @@
 pub mod scsi;
 
 mod aacs;
+mod css;
 mod ld;
-// mod css;   // stage 3 — CSS bus-auth
 
 use scsi::ScsiTransport;
 
@@ -121,6 +121,6 @@ pub fn all_unlockers() -> Vec<Box<dyn Unlocker>> {
     vec![
         Box::new(ld::LibreDrive::new()),
         Box::new(aacs::AacsCert::new()),
-        // Box::new(css::Css::new()),          // stage 3
+        Box::new(css::CssUnlocker::new()),
     ]
 }