Function synthesize 

pub fn synthesize(
    model: &LoadedModel,
    engine: &TtsEngine,
    request: &SynthesisRequest,
    speaker: &[f32],
    seed: u64,
    cancellation: &CancellationToken,
    observer: &dyn SynthesisObserver,
) -> Result<SynthesizedAudio, FttsError>

Run one utterance end to end: text, codes, PCM.

Errors

Engine refusals (admission, budget, cancellation) and model refusals are mapped to their CLI exit classes; a zero-frame generation is reported rather than written out as an empty file.