loadPlaylist
abstract fun loadPlaylist(items: List<OGMediaItem>, startIndex: Int = 0, playWhenReady: Boolean = true)
Loads a playlist: items play in order, each item auto-advancing to the next when it completes (its postrolls included). The UI module's "Up next" countdown card renders during the configurable lead window before each transition; hosts that hide it get silent advancing. PlaybackListener.onPlaylistItemChanged fires on every transition, including the initial item.