Function finish 

fn finish(
    cf_pos: &[f64],
    cf_neg: &[f64],
    mask: &[u8],
    n: usize,
) -> (Vec<f64>, Vec<f64>, Vec<f64>)

Emit the closed-form branch as the final result (χ+ ≥ 0, χ− ≤ 0 signed).