Function load_speakers 

pub fn load_speakers(
    path: Option<&Path>,
    cast_entries: &[CastEntry],
) -> Speakers

Merge CAST-block entries with speakers.json, falling back to the built-ins only when neither exists.

JSON keys always win over auto-derived ones, and a multi-word display name also registers its underscore form. The result is sorted longest-first so compound names match before short ones.