Function compute_operator_norm_sqr 

fn compute_operator_norm_sqr(g: f32, g2: f32, w: f32) -> f32

Compute the squared spectral norm of the TGV operator matrix via power iteration

The operator matrix M is: [0, g, 1 ] [0, 0, g ] [g², w, 0 ]

We compute the largest eigenvalue of M^T * M using power iteration.