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/meta.rs

@@ -290,6 +290,7 @@ fn codec_to_str(c: Codec) -> String {
         Codec::Ac3Plus => "eac3",
         Codec::Lpcm => "lpcm",
         Codec::Pgs => "pgs",
+        Codec::DvdSub => "dvdsub",
         Codec::Unknown(_) => "unknown",
     }
     .into()