open

Opens media and returns the session that reads and decodes it.

Called on the engine's demux worker, so the implementation may block on I/O the way libavformat does. Throwing is how a backend refuses: the engine turns the failure into a typed io.github.yuroyami.kiteplayer.PlaybackError.

MediaItem.io is a factory and must be invoked at most ONCE per call to this member. The engine has already resolved and wrapped whatever the application supplied, so what the factory answers with is this session's reader: close it with the session, and never keep it past one.