Implements SpotifyAudioBackend over a go-librespot sidecar: start() mkfifos
the pipe, spawns the FIFO->48k s16le ffmpeg reader BEFORE go-librespot, writes
config.yml, polls the REST / until ready, then connects the WS event stream.
Maps not_playing/stopped -> trackEnded and metadata -> SpotifyNowPlaying;
play/pause/resume/seek delegate to the REST client. All child_process/fs/REST/WS
seams are injectable so the lifecycle is fully unit-tested without a real binary.
Correction C1: ffmpeg is resolved via getFfmpegCommand() (now exported from
src/audio/player.ts) so the bundled ffmpeg-static fallback is honored in Docker;
the ffmpeg command is overridable via deps.ffmpegCommand for tests.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
