unlock: route_unlock returns a structured UnlockRoute; ctx carries opts

Replace route_unlock's Option<(name, Vid)> with a structured UnlockRoute
{ Unlocked(name, Unlocked) | Failed(UnlockError) | NoMatch } so a single
dispatch serves every caller: drive-prep wants "did anything unlock", and the
AACS cert route (next) needs the FAILURE REASON to render "missing keys" vs
"host cert rejected" instead of collapsing it to a bare None. Only a genuine
SCSI transport fault still returns Err (abort). UnlockCtx gains an optional
ScanOptions (the cert route's host-cert source), and read_mkb_from_drive now
takes &mut dyn ScsiTransport — both prerequisites for the cert handshake to
become an external freemkv-unlock-aacs unlocker. Drive-prep + CSS callers fold
the new outcome; no behavior change.

src/disc/encrypt.rs

@@ -80,7 +80,7 @@ impl AacsCertUnlocker<'_> {
         // MKB generation (best-effort) — forwarded to each source's
         // `host_certs(mkb)` so a source MAY select a generation-appropriate cert
         // (the default impl ignores it). A read failure leaves it `None`.
-        let mkb_gen = aacs::read_mkb_from_drive(session)
+        let mkb_gen = aacs::read_mkb_from_drive(session.scsi_mut())
             .ok()
             .and_then(|m| aacs::mkb_version(&m));