Prim with a cached array queue

This companion provides an executable incremental Prim algorithm over stored adjacency lists. It constructs a minimum spanning tree under the chapter's graph assumptions. Its own generated array-cell work is at most 2n² + 5n + 6|E|, including queue and index-list preparation. No binary-heap operation trace or caller-supplied update counts occur in this interface. The bound counts cell operations and key comparisons, not persistent-array copying or integer bit operations. The ambient index bound n equals |V| when the graph's vertex set is the full finite index universe.