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/mkvstream.rs

@@ -492,6 +492,7 @@ fn parse_track(r: &mut (impl Read + Seek), size: u64) -> io::Result<Option<crate
         "A_DTS" => Codec::Dts,
         "A_PCM/INT/BIG" => Codec::Lpcm,
         "S_HDMV/PGS" => Codec::Pgs,
+        "S_VOBSUB" => Codec::DvdSub,
         _ => Codec::Unknown(0),
     };
     let res = format!("{}p", ph);