Minimum Vertex Cover from Matching

Build a minimum vertex cover of a bipartite graph from a maximum matching, as guaranteed by Koenig's theorem.

First compute a maximum matching. Then mark vertices reachable from unmatched left nodes by alternating paths: unmatched edges go left to right, matched edges go right to left. The cover is every unmarked left node plus every marked right node. Return it as a sorted list of labels like L0 and R1.