craft-audio: MP1/MP2 decoder (#448)

FilmCraft's MPEG audio Layer I/II decoder as `craft_audio::mpa` behind
`mpa-decode`, kept over symphonia's: the same quality where both
decode, and it also decodes the MP2 files ffmpeg writes by default
(mono at 384 kbit/s), which symphonia rejects. `TrackDecoder` and the
native read path take MP1/MP2 in MP4, Matroska and raw streams;
symphonia keeps MP3 only, so its `mp1` and `mp2` features are dropped.

Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>
Co-authored-by: Dylan M. Taylor <dylan@dylanmtaylor.com>

.github/workflows/audio.yml

@@ -39,7 +39,7 @@ jobs:
       # Each feature on its own, so code that needs another feature fails here.
       - name: Clippy, each feature alone
         run: |
-          for f in "" decode decode-native symphonia encode lossless-encode pcm-encode flac-encode peaks aac-decode aac-encode "aac-encode,rayon" opus-decode ac3-decode serde; do
+          for f in "" decode decode-native symphonia encode lossless-encode pcm-encode flac-encode peaks aac-decode aac-encode "aac-encode,rayon" opus-decode ac3-decode mpa-decode serde; do
             echo "::group::features: ${f:-none}"
             cargo clippy -p craft-audio --all-targets --locked --no-default-features ${f:+--features "$f"} -- -D warnings
             echo "::endgroup::"