Local resolve: MK-pool brute via km_verifies (path 2.5)

keydb stores Media Keys per-disc, but an MK is MKB-scoped (shared across a
pressing/MKB-family). A disc whose own hash/VID isn't keyed can still resolve
if any stored MK verifies against its MKB. New path 2.5 (between PK and the
VID lookup) collects the distinct MK pool from the providers, km_verifies each
against the disc MKB, and on a UNIQUE pass derives VUK (with the disc VID) then
the UK — matching the online resolver's behavior so local keydb mode resolves
the same discs (e.g. an MK present in keydb under a sibling pressing).
km_verifies is one AES-D + magic check per candidate (cheap). Adds
KeyProvider::media_keys() + a path-2.5 unit test.

src/aacs/provider.rs

@@ -48,6 +48,14 @@ pub trait KeyProvider: Send + Sync {
         Vec::new()
     }
 
+    /// Every Media Key this provider holds, regardless of which disc it was
+    /// filed under. An MK is MKB-scoped (shared across a pressing/MKB-family),
+    /// so the resolver can verify each against the disc's MKB (`km_verifies`)
+    /// and resolve a disc whose own hash/VID isn't directly keyed.
+    fn media_keys(&self) -> Vec<[u8; 16]> {
+        Vec::new()
+    }
+
     /// AACS host certificates (with their private keys) for drive
     /// authentication. Multiple in case some are revoked.
     fn host_certs(&self) -> Vec<HostCert> {
@@ -85,6 +93,15 @@ impl Providers<'_> {
         self.0.iter().flat_map(|p| p.processing_keys()).collect()
     }
 
+    /// Union of distinct Media Keys across every provider, for the MK-pool
+    /// brute (`km_verifies` against the disc's MKB).
+    pub fn media_keys(&self) -> Vec<[u8; 16]> {
+        let mut v: Vec<[u8; 16]> = self.0.iter().flat_map(|p| p.media_keys()).collect();
+        v.sort_unstable();
+        v.dedup();
+        v
+    }
+
     /// Union — gather host certs from every provider. Not yet wired into
     /// the SCSI handshake (which still reads `KeyDb.host_certs` directly);
     /// kept here so a provider-aware handshake refactor is a drop-in.