alice writes M1 on node-1
bob writes M2 on node-2
  M1=0322eb82-e50a-4583-866e-09f7b03a15c2 M2=19a78150-a866-4ead-9a12-ec827c7bd50d
settle 5s for pre-link replication
alice links M1 -> M2 with relation=related_to
  link POST returned HTTP 201
settle 8s for link fanout
charlie queries links of M1 on node-3
  charlie sees M1->M2 link: 1 (expected >=1)