/// Hypercubic neighbor offsets: one unit vector per dimension.
///
/// For `n_dims = 3` this returns `[[1,0,0], [0,1,0], [0,0,1]]`.
/// Triangular-lattice neighbor offsets (2D only).
///
/// Returns `[[1,0], [0,1], [1,-1]]` — three forward directions giving
/// coordination number 6.