Function run 

pub fn run(
    left: &[Step],
    right: &[Step],
    left_path: &Path,
    right_path: &Path,
    left_format: &str,
    right_format: &str,
    no_cost: bool,
) -> Result<()>

Launch the diff TUI. Caller supplies the already-loaded step vecs so main.rs can reuse the existing load_session path and error handling. Returns when the user quits with q or Esc.