Give every audio and video codec its own registered Matroska CodecID

MkvTrack::audio's catch-all was `_ => CODEC_AC3`, and ebml.rs defined no
A_AAC, A_MPEG/L2, A_MPEG/L3, A_FLAC or A_OPUS constant at all. A CodecID
names the payload, so any of those codecs was written into the MKV declaring
AC-3 while carrying something else — a player either refuses the track or
decodes noise.

Reachable through two ordinary paths, both verified: ifo.rs:577 maps DVD
audio_coding_mode 3 to Codec::Mp2, so a DVD with MPEG audio muxed to mkv://
produced a track declaring A_AC3 over MP2 bytes; and mp4/read.rs:479 maps the
`mp4a` sample entry to Codec::Aac for an mp4:// source. codec/mod.rs already
has working parsers for MP2, MP3, AAC, FLAC and Opus, so the pipeline carried
these codecs end to end and only the container label was wrong.

MkvTrack::video had the same shape: `_ => CODEC_MPEG2` announced Codec::Mpeg1
and Codec::Av1 as MPEG-2 video. V_MPEG1 and V_AV1 added.

Both catch-alls stay, because MkvTrack::audio/video return Self and have no
error channel, but they are now reachable only by a non-audio/non-video or
Unknown codec routed there in error. Two tests enumerate every real codec of
each kind and cross-check each against Codec::kind(), so a codec added to the
enum later cannot silently inherit another codec's ID.

Both proven red first: Aac declared A_AC3 before the fix.

src/mux/ebml.rs

@@ -582,11 +582,28 @@ pub const CODEC_HEVC: &str = "V_MPEGH/ISO/HEVC";
 pub const CODEC_H264: &str = "V_MPEG4/ISO/AVC";
 pub const CODEC_VC1: &str = "V_MS/VFW/FOURCC";
 pub const CODEC_MPEG2: &str = "V_MPEG2";
+/// MPEG-1 Video. Distinct from V_MPEG2: a decoder selects its bitstream
+/// parser from this ID.
+pub const CODEC_MPEG1: &str = "V_MPEG1";
+/// AV1. CodecPrivate carries the AV1CodecConfigurationRecord.
+pub const CODEC_AV1: &str = "V_AV1";
 pub const CODEC_AC3: &str = "A_AC3";
 pub const CODEC_EAC3: &str = "A_EAC3";
 pub const CODEC_TRUEHD: &str = "A_TRUEHD";
 pub const CODEC_DTS: &str = "A_DTS";
 pub const CODEC_PCM_BE: &str = "A_PCM/INT/BIG";
+/// AAC. The generic registered ID; the AudioSpecificConfig travels in
+/// CodecPrivate, so no profile suffix is needed (and the `A_AAC/MPEG4/*`
+/// suffixed forms are legacy).
+pub const CODEC_AAC: &str = "A_AAC";
+/// MPEG-1/2 Audio Layer II — DVD audio_coding_mode 3.
+pub const CODEC_MP2: &str = "A_MPEG/L2";
+/// MPEG-1/2 Audio Layer III.
+pub const CODEC_MP3: &str = "A_MPEG/L3";
+/// FLAC. CodecPrivate carries the STREAMINFO metadata block.
+pub const CODEC_FLAC: &str = "A_FLAC";
+/// Opus. CodecPrivate carries the OpusHead identification header.
+pub const CODEC_OPUS: &str = "A_OPUS";
 pub const CODEC_PGS: &str = "S_HDMV/PGS";
 pub const CODEC_VOBSUB: &str = "S_VOBSUB";
 

src/mux/mkv.rs

@@ -368,6 +368,12 @@ impl MkvTrack {
             Codec::Hevc => ebml::CODEC_HEVC,
             Codec::Vc1 => ebml::CODEC_VC1,
             Codec::Mpeg2 => ebml::CODEC_MPEG2,
+            Codec::Mpeg1 => ebml::CODEC_MPEG1,
+            Codec::Av1 => ebml::CODEC_AV1,
+            // Every video codec this crate can produce is named above. The
+            // remaining arm is reached only by a non-video or Unknown codec
+            // routed here in error — see the audio counterpart for why this
+            // keeps a fallback rather than erroring.
             _ => ebml::CODEC_MPEG2,
         };
         // An Unknown resolution has no real dimensions — emit (0, 0) so the
@@ -492,6 +498,17 @@ impl MkvTrack {
             Codec::TrueHd => ebml::CODEC_TRUEHD,
             Codec::DtsHdMa | Codec::DtsHdHr | Codec::Dts => ebml::CODEC_DTS,
             Codec::Lpcm => ebml::CODEC_PCM_BE,
+            Codec::Aac => ebml::CODEC_AAC,
+            Codec::Mp2 => ebml::CODEC_MP2,
+            Codec::Mp3 => ebml::CODEC_MP3,
+            Codec::Flac => ebml::CODEC_FLAC,
+            Codec::Opus => ebml::CODEC_OPUS,
+            // Every audio codec this crate can produce is named above. The
+            // remaining arm is reached only by a non-audio or Unknown codec
+            // routed here in error; A_AC3 is the historical fallback and is
+            // wrong for such a track, but MkvTrack::audio has no error channel —
+            // `every_audio_codec_gets_its_own_registered_codec_id` pins the real
+            // codecs so a newly-added one cannot silently land here instead.
             _ => ebml::CODEC_AC3,
         };
         // Unknown sample rate / channel layout: emit 0 so the serializer omits
@@ -2380,6 +2397,98 @@ mod tests {
         );
     }
 
+    #[test]
+    fn every_audio_codec_gets_its_own_registered_codec_id() {
+        // A CodecID names the payload. Falling through to A_AC3 for a codec the
+        // match had no arm for writes a track declaring AC-3 while carrying
+        // something else entirely — a player either refuses the track or decodes
+        // noise. Every audio codec this crate can produce must map to its own
+        // registered Matroska ID.
+        //
+        // Reachable: ifo.rs maps DVD audio_coding_mode 3 to Codec::Mp2, and
+        // mp4/read.rs maps the `mp4a` sample entry to Codec::Aac, so a DVD with
+        // MPEG audio and an mp4:// AAC source both hit this path.
+        for (codec, want) in [
+            (Codec::Ac3, "A_AC3"),
+            (Codec::Ac3Plus, "A_EAC3"),
+            (Codec::TrueHd, "A_TRUEHD"),
+            (Codec::Dts, "A_DTS"),
+            (Codec::DtsHdHr, "A_DTS"),
+            (Codec::DtsHdMa, "A_DTS"),
+            (Codec::Lpcm, "A_PCM/INT/BIG"),
+            (Codec::Aac, "A_AAC"),
+            (Codec::Mp2, "A_MPEG/L2"),
+            (Codec::Mp3, "A_MPEG/L3"),
+            (Codec::Flac, "A_FLAC"),
+            (Codec::Opus, "A_OPUS"),
+        ] {
+            let got = MkvTrack::audio(&audio_stream(codec)).codec_id;
+            assert_eq!(
+                got, want,
+                "{codec:?} must declare {want}, got {got} — a wrong CodecID \
+                 mislabels the payload"
+            );
+        }
+        // Every Codec of audio kind must be covered above: a new audio codec
+        // added to the enum must not silently inherit some other codec's ID.
+        let covered = [
+            Codec::Ac3,
+            Codec::Ac3Plus,
+            Codec::TrueHd,
+            Codec::Dts,
+            Codec::DtsHdHr,
+            Codec::DtsHdMa,
+            Codec::Lpcm,
+            Codec::Aac,
+            Codec::Mp2,
+            Codec::Mp3,
+            Codec::Flac,
+            Codec::Opus,
+        ];
+        for codec in covered {
+            assert_eq!(
+                codec.kind(),
+                crate::disc::CodecKind::Audio,
+                "{codec:?} is listed as audio here but kind() disagrees"
+            );
+        }
+    }
+
+    #[test]
+    fn every_video_codec_gets_its_own_registered_codec_id() {
+        // Same defect class as the audio counterpart: the catch-all declared
+        // V_MPEG2 for Codec::Mpeg1 and Codec::Av1, so an AV1 track was announced
+        // to the decoder as MPEG-2 video.
+        for (codec, want) in [
+            (Codec::H264, "V_MPEG4/ISO/AVC"),
+            (Codec::Hevc, "V_MPEGH/ISO/HEVC"),
+            (Codec::Vc1, "V_MS/VFW/FOURCC"),
+            (Codec::Mpeg2, "V_MPEG2"),
+            (Codec::Mpeg1, "V_MPEG1"),
+            (Codec::Av1, "V_AV1"),
+        ] {
+            let v = VideoStream {
+                pid: 0xE0,
+                codec,
+                resolution: Resolution::R1080p,
+                frame_rate: crate::disc::FrameRate::F24,
+                hdr: HdrFormat::Sdr,
+                color_space: ColorSpace::Bt709,
+                display_aspect: None,
+                secondary: false,
+                label: String::new(),
+                measured_cicp: None,
+            };
+            let got = MkvTrack::video(&v).codec_id;
+            assert_eq!(got, want, "{codec:?} must declare {want}, got {got}");
+            assert_eq!(
+                codec.kind(),
+                crate::disc::CodecKind::Video,
+                "{codec:?} is listed as video here but kind() disagrees"
+            );
+        }
+    }
+
     #[test]
     fn dolby_vision_config_profile7() {
         // dvcC for disc Profile 7 dual-layer: version 1.0, profile 7, all of