keydb: union stored UKs + VUK/MK-derived unit keys (dedup by value)

unit_keys_from now loads all stored unit keys AND the VUK/MK-derived ones, deduping by key value instead of first-hit, so a disc with partial stored UKs plus a VUK yields every declared CPS unit key. Plus keydb.cfg format round-trip (to_keydb_cfg inverse of parse).

src/keydb_format.rs

@@ -1475,7 +1475,7 @@ mod tests {
         let original = std::fs::read(&unit_path).unwrap();
         assert_eq!(original.len(), libfreemkv::aacs::ALIGNED_UNIT_LEN);
         assert!(
-            libfreemkv::aacs::is_aacs_scrambled(&original),
+            libfreemkv::aacs::ts_sync_destroyed(&original),
             "Unit should be encrypted"
         );