Implements the Stage-2 SpotifyAudioBackend over Rust librespot: spawns
librespot with --backend pipe (no --device => s16le/44100/2 on stdout, no
--passthrough), pipes stdout -> ffmpeg (44100->48000 s16le), waits for the
Connect device to register before emitting "ready", and controls playback
(transfer/play/pause/resume/seek) plus track-end/position/metadata via a
polled SpotifyConnectApi. child_process/connect/oauth/ffmpeg injected for
fully mocked, no-network unit tests (Windows-targeted; not e2e without Premium).
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>