Function r2primenet_from_magnitude 

pub fn r2primenet_from_magnitude(
    magnitude: &[f64],
    echo_times: &[f64],
    mask: &[u8],
    grid: &Grid,
    b0_tesla: f64,
    model_onnx: &[u8],
    norm: &R2PrimeNetNorm,
    params: &R2PrimeNetParams,
    progress: impl FnMut(usize, usize),
) -> Result<(Vec<f64>, Vec<f64>), OnnxError>

Convenience: fit R2* from multi-echo magnitude with ARLO, then run r2primenet.

magnitude is voxel-major (n_voxels, n_echoes) and echo_times are in seconds (ARLO requires ≥3 increasing echo times; other echo times are OnnxError::Domain); R2* is 0 outside mask. Returns (r2prime_hz, r2star_hz) so the caller can keep the intermediate R2* map.