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/keys.rs

@@ -1103,6 +1103,30 @@ fn resolve_keys_classical(ctx: &ResolveContext<'_>, version: AacsVersion) -> Opt
                 return Some(build(Some(vuk), derive_uks(&vuk), 2));
             }
             tracing::warn!(target: "freemkv::disc", phase = "resolve_keys_path2_miss", pk_count = all_pks.len(), "PK derivation failed");
+
+            // Path 2.5: MK-pool brute. 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. Try every distinct MK via km_verifies;
+            // a UNIQUE pass is this disc's Km → derive VUK (needs VID) → UK.
+            // km_verifies is one AES-D + magic check per candidate (cheap).
+            let mks = providers.media_keys();
+            let mut mk_hits: Vec<[u8; 16]> = Vec::new();
+            for mk in &mks {
+                if probe::km_verifies(mkb, mk) && !mk_hits.contains(mk) {
+                    mk_hits.push(*mk);
+                    if mk_hits.len() > 1 {
+                        break; // ambiguous — bail to avoid a wrong key
+                    }
+                }
+            }
+            if mk_hits.len() == 1 {
+                let vuk = derive_vuk(&mk_hits[0], ctx.volume_id);
+                tracing::warn!(target: "freemkv::disc", phase = "resolve_keys_path2_5_hit", mk_pool = mks.len(), "media key from keydb MK-pool brute (km_verifies)");
+                // Same class as path 3 (KEYDB MK → derived VUK).
+                return Some(build(Some(vuk), derive_uks(&vuk), 3));
+            }
+            tracing::warn!(target: "freemkv::disc", phase = "resolve_keys_path2_5_miss", mk_pool = mks.len(), mk_hits = mk_hits.len(), "MK-pool brute: no unique verifying MK");
         } else {
             tracing::warn!(target: "freemkv::disc", phase = "resolve_keys_no_mkb", "no MKB; paths 1/2 skipped");
         }
@@ -1954,6 +1978,68 @@ mod tests {
         );
     }
 
+    #[test]
+    fn resolve_keys_path2_5_mk_pool_brute_resolves_unkeyed_disc() {
+        // The Dunkirk case: this disc's own hash/VID are NOT in keydb, but its
+        // Media Key IS — filed under a sibling disc that shares its MKB. Path
+        // 2.5 must km_verifies that MK against the MKB and resolve.
+        use super::super::decrypt::aes_ecb_encrypt as enc;
+
+        let km = [0x11u8; 16];
+        let vid = [0x22u8; 16];
+
+        // MKB: 0x10 type/version + 0x86 verify record whose mk_dv decrypts under
+        // km to the AACS verify magic, so km_verifies(mkb, km) == true.
+        let mut vd = [0u8; 16];
+        vd[..8].copy_from_slice(&[0x01, 0x23, 0x45, 0x67, 0x89, 0xAB, 0xCD, 0xEF]);
+        let mk_dv = enc(&km, &vd);
+        let mut mkb = mkb_record(0x10, &[0, 0, 0, 0x20, 0, 0, 0, 0x4D]);
+        mkb.extend_from_slice(&mkb_record(0x86, &mk_dv));
+        assert!(
+            probe::km_verifies(&mkb, &km),
+            "fixture: km must verify the MKB"
+        );
+
+        // This disc's inf (its hash will NOT be in keydb).
+        let uk_ro = minimal_unit_key_ro();
+
+        // keydb: a SIBLING disc carries our km, keyed by the sibling's own
+        // hash + VID (neither matches THIS disc) — so only the MK-pool brute
+        // (km_verifies) can find it.
+        let mut keydb = KeyDb::empty();
+        keydb.disc_entries.insert(
+            "0xsibling".to_string(),
+            DiscEntry {
+                disc_hash: "0xsibling".to_string(),
+                title: "sibling".to_string(),
+                media_key: Some(km),
+                disc_id: Some([0x99u8; 16]),
+                vuk: None,
+                unit_keys: Vec::new(),
+            },
+        );
+
+        let providers: &[&dyn super::super::KeyProvider] = &[&keydb];
+        let ctx = ResolveContext {
+            unit_key_ro: &uk_ro,
+            content_cert: None,
+            volume_id: &vid,
+            providers,
+            mkb: Some(&mkb),
+        };
+        let resolved = resolve_keys_v1(&ctx)
+            .expect("MK-pool brute (path 2.5) must resolve a disc whose MK is in keydb");
+        assert_eq!(
+            resolved.key_source, 3,
+            "MK-pool brute is the KEYDB-derived class"
+        );
+        assert_eq!(
+            resolved.vuk,
+            Some(derive_vuk(&km, &vid)),
+            "VUK must derive from the verified Km + this disc's VID"
+        );
+    }
+
     #[test]
     fn test_content_cert_parse() {
         // AACS 1.0 cert