Function update_hessian_powell 

pub fn update_hessian_powell(
    hessian: &DMatrix<f64>,
    delta_x: &DVector<f64>,
    delta_g: &DVector<f64>,
) -> DMatrix<f64>

Powell symmetric rank-one update.

Implements the Powell/SR1 formula:

H_new = H + (Δg - H·Δx)(Δg - H·Δx)^T / [(Δg - H·Δx)·Δx]

This update can handle negative curvature, making it suitable for transition state searches.