Function minimum_spanning_tree 

pub fn minimum_spanning_tree(
    graph: &Graph,
    weight_attr: &str,
) -> MinimumSpanningTreeResult

Computes the minimum spanning tree using Kruskal’s algorithm.

Reads edge weights from the attribute weight_attr (parsed as f64). Missing or unparseable weights default to 1.0. Returns edges in deterministic sorted order (min(u,v), max(u,v)).