100% codec coverage + disc/ and aacs/ module refactors

Codec coverage (DVD + BD + UHD):
- E-AC-3 (Dolby Digital Plus): bsid detection, frame size calc — 8 tests
- DTS-HD MA/HR: extension substream (0x64582025) detection — 8 tests
- LPCM: BD header skip, raw PCM extraction — 6 tests
- DVD VobSub subtitles: passthrough parser — 5 tests
- Dolby Vision: verified RPU NAL type 62 preserved in HEVC — 1 test

Module refactors:
- disc.rs → disc/mod.rs + bluray.rs + dvd.rs + encrypt.rs
- aacs/mod.rs (1661 lines) → mod.rs (21) + keydb.rs + keys.rs + decrypt.rs
- All public APIs preserved, all tests pass

270 tests total, 0 failures.

src/mux/mkv.rs

@@ -80,9 +80,13 @@ impl MkvTrack {
     }
 
     pub fn subtitle(s: &SubtitleStream) -> Self {
+        let codec_id = match s.codec {
+            Codec::DvdSub => "S_VOBSUB",
+            _ => "S_HDMV/PGS",
+        };
         Self {
             track_type: ebml::TRACK_TYPE_SUBTITLE,
-            codec_id: "S_HDMV/PGS",
+            codec_id,
             language: s.language.clone(),
             name: String::new(),
             codec_private: None,