Function merge_regions_post 

fn merge_regions_post(
    phase: &mut [f64],
    visited: &mut [u8],
    num_regions: u8,
    weights: &[u8],
    nx: usize,
    ny: usize,
    nz: usize,
) -> Vec<u8> 

Merge adjacent regions by computing inter-region phase offsets.

Calculates weighted phase offset between adjacent region pairs, then iteratively merges regions starting from the largest. Matches ROMEO.jl merge_regions!.