craft-audio: Opus encoder

EffectCraft's `effectcraft-opusenc` as `opus::encoder` (and
`opus::Encoder`) behind `opus-encode`: SILK, hybrid and CELT, 48 kHz,
20 ms frames, mono or stereo. The constructors and `encode_float`
return `Result` instead of panicking. `Bandwidth` is one type for the
decoder and the encoder, and the two `Mode` enums convert into each
other. `encode` now turns on `opus-encode`.

The encoder tests (modes, ffmpeg's two Opus decoders as oracles) and
`TrackDecoder` on Opus without an `OpusHead` come with it.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

.github/workflows/audio.yml

@@ -44,7 +44,7 @@ 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 mpa-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 opus-encode 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::"