Function spectral_granger_causality 

pub fn spectral_granger_causality(
    trains: &[&[i32]],
    bin_size: usize,
    order: usize,
    n_freqs: usize,
) -> (Vec<f64>, usize)

Spectral Granger causality (Geweke 1982). Returns (d × d × n_freqs) as flat Vec, row-major in [i][j][f] order.