cuesFor method
The parsed CaptionCues for caption source — synchronous by contract,
so the caption layer can pick the active cue at the current frame.
Only valid after preResolveCaptions completed for source;
implementations throw on an unresolved source naming it.
Implementation
@override
List<CaptionCue> cuesFor(CaptionSource source) => throw FluvieRenderException(
'NoMediaResolver has no parsed captions for "$source". Wire a '
'MediaRepository through mediaResolverProvider to render captions.',
);