Function treat_dialogue 

fn treat_dialogue(
    segment: Segment,
    plan: &StemPlan,
    cast: &IndexMap<String, Voice>,
    span_map: &IndexMap<i64, Vec<&'static str>>,
    vintage_scenes: &[String],
    dialogue_only: bool,
) -> Result<Segment>

Dialogue treatment shared by the foreground and the dialogue layer.