OptionalCaptionsUploaded File.ID for captions.vtt when captions were generated and stored.
OptionalCaptionsGenerated WebVTT document when Audio.Captions !== 'None'.
Synthesized audio clips in script order.
Structured errors and warnings collected during synthesis, storage, and timeline fitting.
Deep copy of the input spec with fitted beats, narration timings, cues, and film duration.
Result returned by RunNarrationPipeline.