Function player_set_subtitle_track 

pub async fn player_set_subtitle_track(
    player: State<'_, PlayerStateWrapper>,
    stream_index: Option<i32>,
) -> Result<PlayerStatus, String>

Set (or clear, with None) the active subtitle track on a native backend.

On Android this indexes ExoPlayer’s text track groups — i.e. the position of the sideloaded MediaItem.SubtitleConfiguration, not the Jellyfin stream index. The HTML5 path never reaches here; it toggles its own <track> children. libmpv implements neither, leaving the trait default in place.

TRACES: UR-020 | IR-018, DR-023