Function bisect_gradient 

fn bisect_gradient<F>(
    model_prices: &mut F,
    markets: &[f64],
) -> Result<f64, CalibrationError>
where
    F: FnMut(f64) -> Result<Vec<Jet1<f64>>, CalibrationError>,

Bisection on the least-squares gradient over the bracket — the fallback when Gauss-Newton keeps leaving it.