Function solve_call_iv 

fn solve_call_iv(
    spot: f64,
    strike: f64,
    tte: f64,
    r: f64,
    target: f64,
    iv_hint: f64,
) -> f64

Find σ in [IV_FLOOR, iv_hint] such that BS call price ≈ target. BS call is monotone increasing in σ, so bisection converges. If the lower bound’s price already exceeds target (target below intrinsic), returns IV_FLOOR.