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>