alice writes M1 on node-1
bob writes M2 on node-2
  M1=cd33820d-9df6-4632-bf6b-0c06ce6c9b85 M2=65fc0f35-27a9-41c0-a458-f3eb839c89fa
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)