Function convolve_1d_direction 

fn convolve_1d_direction(
    data: &[f64],
    nx: usize,
    ny: usize,
    nz: usize,
    kernel: &[f64],
    direction: char,
) -> Vec<f64>

Apply 1D convolution along specified axis with replicate padding Matches MATLAB’s imgaussian which uses ‘replicate’ boundary handling