Function sti_fastqsm 

fn sti_fastqsm(
    p_k: &[Complex64],
    m: &[f64],
    inside: &[usize],
    dk: &[f64],
    n: [usize; 3],
    fft: &dyn Fn(&mut [Complex64]),
    ifft: &dyn Fn(&mut [Complex64]),
) -> Vec<f64>

STI’s FastQSM on the padded grid, given F φ (unshifted k-space). Returns the masked estimate, rescaled to the TKD (threshold 1/8) solution by least squares inside the mask.