Function krawczyk_operator 

pub fn krawczyk_operator(
    f_mid: f64,
    y: f64,
    df_iv: Interval,
    iv: Interval,
) -> Interval

Krawczyk operator for root verification.

Given f : ℝ → ℝ, approximate Jacobian inverse y ≈ 1/f'(m), and interval iv, computes K = m - y*f(m) + (I - y*f'(iv))*(iv - m).

If K ⊆ iv then iv contains exactly one root. If K ∩ iv = ∅ then iv contains no root.