Function fit_r2star_weighted_loglinear 

fn fit_r2star_weighted_loglinear(
    magnitude: &[f64],
    echo_times: &[f64],
    mask: &[u8],
    n_voxels: usize,
) -> Vec<f64>

R2* (Hz) by magnitude-weighted log-linear least squares over echoes.

Weight ∝ magnitude² (SNR) so noisy late echoes don’t dominate — the QSM-CI reference’s robust stand-in for ARLO. R2* = −slope of log(magnitude) vs TE, clamped to ≥ 0.