aacs: source OEM host certs from keysource layer

Complete the OEM/AACS cert baseline so host certs are a KeySource output,
never compiled in. With an unlocker present the OEM route is unused
(unlocker_read_volume_id short-circuits); without one, the cert handshake
runs when a keysource supplies a host cert and fails gracefully when none
does.

- KeySource trait gains host_certs() (default empty), reusing the existing
  aacs::HostCert type. A source holds certs as its second kind of AACS
  material alongside decryption keys.
- ScanOptions gains key_sources so the handshake can collect certs across
  the app's keysource layer, unioned with DriveCredentials.
- do_handshake_cert collects certs via collect_host_certs (credentials +
  every key source). Zero certs from any source now returns the new
  graceful Error::AacsNoHostCert (code 7024, sentinel <no host cert>)
  instead of silently skipping; resolution still falls back to the
  path-1 disc-hash -> VUK lookup, which drops the error on a hit.
- error.rs: add E_AACS_NO_HOST_CERT / Error::AacsNoHostCert, wired into
  code(), Display, and the round-trip + sentinel tests.

HandshakeResult { volume_id, read_data_key } unchanged: the cert path
still yields both the VID and the bus key.

src/keysource.rs

@@ -11,6 +11,7 @@
 //! out of the library while all key *mechanism* (the AACS derivation chain)
 //! stays in it.
 
+use crate::aacs::HostCert;
 use crate::disc::Key;
 
 /// The public AACS inputs a key source needs to look a disc up. Captured at
@@ -92,6 +93,25 @@ pub trait KeySource {
     fn errored(&self) -> bool {
         false
     }
+
+    /// The AACS host certificate(s) this source can supply for the live-drive
+    /// SCSI mutual-auth handshake (the OEM/AACS baseline route). A host cert is
+    /// the *second* kind of AACS material a source may hold, distinct from the
+    /// decryption keys handed out by [`KeySource::next_key`]: it unlocks the
+    /// authenticated bus so the drive will report the Volume ID and bus key,
+    /// whereas the keys decrypt content once the disc is read.
+    ///
+    /// Returned, never compiled in: a host cert is **perishable** — it can be
+    /// revoked on a given drive's Host Revocation List (carried forward by newer
+    /// discs' MKBs), so it must be rotatable, hence served by a source rather
+    /// than baked into the binary. A source that holds no cert (a mapfile, or an
+    /// online service whose cert-serving isn't yet designed) returns the empty
+    /// vec — the default. The handshake collects across every source and tries
+    /// each candidate; with no candidate from any source the OEM route fails
+    /// gracefully ([`crate::Error::AacsNoHostCert`]), it never panics.
+    fn host_certs(&self) -> Vec<HostCert> {
+        Vec::new()
+    }
 }
 
 #[cfg(test)]