Function update_hessian_by_method 

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

Updates the Hessian matrix using the specified method.

Dispatches to the appropriate update formula based on the HessianMethod:

• DirectPsb: PSB (Powell-Symmetric-Broyden) rank-2 update on direct H
• InverseBfgs: BFGS inverse Hessian update (legacy)
• Bofill: Bofill weighted update for saddle-point-like crossings
• Powell: Powell symmetric rank-one (SR1) update
• BfgsPowellMix: Adaptive BFGS/Powell blend with Bofill weighting

Arguments

• hessian - Current Hessian matrix (direct or inverse depending on method)
• delta_x - Step vector (x_new - x_old) in A
• delta_g - Gradient difference (g_new - g_old) in Ha/A
• method - Hessian update method to use

Returns

Updated Hessian matrix.