Function reconstruction_error 

pub fn reconstruction_error(
    a: &SparseColMat<usize, f64>,
    reference: &ReferenceFactorization,
) -> f64

Compute the relative reconstruction error of a factorization.

Returns ||P^T A P - L D L^T||_F / ||A||_F where L, D, P come from the reference factorization. The Frobenius norm is computed via faer’s norm_l2() method on Mat, which returns ||M||_F (not the spectral 2-norm).

Returns 0.0 if ||A||_F is zero.