Function subtitle_supports_external_delivery 

pub fn subtitle_supports_external_delivery(codec: Option<&str>) -> bool

Whether a subtitle in this format can reach the app as a sidecar it draws itself — the same verdict as subtitle_forces_burn_in, from the reader’s side, and the one a subtitle picker needs.

Since the app asks for burn-in nowhere (see playback_subtitle_stream_index), a format that only burn-in could deliver is one it can never display. An unnamed format is treated as undeliverable rather than guessed at: offering a track and drawing nothing is worse than not offering it.

TRACES: UR-020 | DR-176 | UT-168