Function build_interfaces 

fn build_interfaces(
    phase: &[f64],
    region_of: &[u32],
    n_regions: usize,
    dims: (usize, usize, usize),
) -> Vec<BTreeMap<u32, Interface>>

Accumulate N_AB and P_AB for every interface, as a symmetric adjacency list.

Only the three forward neighbours are visited, so each interfacing voxel pair is counted exactly once — which is what N_AB in eq 6 is defined to be.