Function minimum_spanning_tree 

pub fn minimum_spanning_tree<V>(
    q: &GraphQuery<V>,
    weight: &TraversalWeight<V>,
) -> Vec<Pattern<V>>
where
    V: GraphValue + Clone,
    V::Id: Clone + Eq + Hash + Ord,

Minimum spanning tree using Kruskal’s algorithm with path-compression union-find.

• Edge cost is min(forward_cost, backward_cost).
• Edges with INFINITY cost in both directions are excluded.
• Returns the subset of nodes that are included in the MST.