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/disc/mod.rs

@@ -1135,7 +1135,22 @@ pub struct DriveCredentials {
 pub struct ScanOptions {
     /// Host credentials for the live-drive AACS handshake. `None` for ISO
     /// scans, or a live drive where cert auth should be skipped.
+    ///
+    /// Host certs may ALSO be supplied through [`Self::key_sources`]: the
+    /// handshake unifies certs from both, so the app can pass its already-built
+    /// keysource layer rather than (or in addition to) pre-extracting certs into
+    /// `DriveCredentials`. Either route is keysource-served — certs are never
+    /// compiled into the library.
     pub credentials: Option<DriveCredentials>,
+    /// The application's key-source layer. The handshake collects host certs
+    /// across these (via [`crate::KeySource::host_certs`]) for the OEM/AACS
+    /// cert-auth route, unioned with [`Self::credentials`]. Empty by default —
+    /// an ISO scan supplies none, and a live-drive caller that pre-extracted
+    /// certs into `credentials` may leave it empty too. The library still
+    /// resolves NO keys from these at scan time; they are consulted only for
+    /// their host certs here (key *resolution* stays out-of-band via
+    /// `Disc::decrypt_with`).
+    pub key_sources: Vec<Box<dyn crate::KeySource>>,
     /// Optional cooperative-cancellation token. When set, long scan-time
     /// loops (notably the CSS known-plaintext crack, which can scan up to
     /// 50_000 sectors on a live DVD) poll it and bail out cleanly so a