Function gpu_cg_solver 

pub fn gpu_cg_solver(
    mat: &SparseMatrixGpu,
    b: &[f64],
    max_iter: usize,
    tol: f64,
) -> (Vec<f64>, usize, f64)

Conjugate gradient solver for symmetric positive-definite systems A·x = b.

Returns (x, iterations, final_residual_norm). Terminates when the relative residual ‖r‖/‖b‖ < tol or after max_iter steps.