Decode a library track to the PCM format AirPlay/RAOP wants: 44.1 kHz, 16-bit,
stereo, little-endian, via ffmpeg. Returns the raw interleaved s16le bytes,
which AirPlay.Alac chops into 352-sample frames.
Summary
Functions
Split PCM into frames-sample stereo chunks (last one zero-padded).
Decode path to interleaved s16le stereo @44.1kHz, or {:error, reason}.