Function get_boundary_edges 

pub fn get_boundary_edges<T, V>(
    treetn: &TreeTN<T, V>,
    region: &[V],
) -> Result<Vec<BoundaryEdge<T, V>>>
where
    T: TensorLike,
    T::Index: IndexLike,
    V: Clone + Hash + Eq + Send + Sync + Debug,

Get all boundary edges for a given region in a TreeTN.

Returns edges connecting nodes inside the region to neighbors outside the region. This is useful for maintaining stable bond IDs across updates (e.g., for environment cache consistency).

Arguments

• treetn - The TreeTN to analyze
• region - Nodes that are inside the region

Returns

Vector of boundary edges, each containing the node in region, neighbor outside, and bond index.