Function build_neighbor_matrix 

pub fn build_neighbor_matrix(
    n_vertices: usize,
    faces: &[[usize; 3]],
    max_neighbors: usize,
) -> (Vec<Vec<usize>>, Vec<usize>)

Build neighbor matrix for vectorized neighbor lookups

Returns (neighbor_matrix, neighbor_counts) where:

• neighbor_matrix[i] contains the indices of vertex i’s neighbors (padded with usize::MAX)
• neighbor_counts[i] is the number of valid neighbors for vertex i