### The composing sink trims an Ogg Opus piece to its pre-skip and end

- **`wai-web/compose/`**: an alternative of `wai.audio.opus` routed to the user agent is an Ogg
  Opus stream whose pre-skip and end (RFC 7845 §4) a decoder handed its packets does not apply:
  `compose.js` drops the `OpusHead` pre-skip (less what the decoder dropped itself), ends the
  samples at the last page's granule position, and holds an Opus floor's piece (packaging §7.9)
  to exactly its piece's samples (`compose_frames` otherwise). `run.mjs` plays
  `pack-conformance/multi/multi-opus-01` without FLAC: where the engine's `AudioDecoder` takes
  Opus every piece plays its Opus floor and comes to exactly the piece's samples.