Function player_switch_audio_track 

pub async fn player_switch_audio_track(
    player: State<'_, PlayerStateWrapper>,
    repository_manager: State<'_, RepositoryManagerWrapper>,
    repository_handle: String,
    stream_index: i32,
    array_index: i32,
    use_html5: bool,
    current_position: Option<f64>,
    media_source_id: Option<String>,
) -> Result<AudioTrackSwitchResponse, String>

Switch audio track - handles both HTML5 (stream reload) and native (direct switch) Note: Frontend should handle saving series preferences after this command succeeds

The split is the requirement: an HTML5 <video> element cannot be told to change audio track, so the stream is re-opened at the chosen AudioStreamIndex and the frontend seeks the reloaded element back to position; a native backend (ExoPlayer) switches in place by track-group index. libmpv implements neither — it is the audio-only backend here and leaves PlayerBackend::set_audio_track at its not_implemented() default, which is why IR-019 is met by these two paths rather than by MPV.

TRACES: UR-021 | IR-019, DR-024