alice writes M1 on node-1
bob writes M2 on node-2
  M1=fc1a9fab-d91c-4bbb-904a-169a12cd5b33 M2=3b3512f9-8555-4be0-94eb-97173b0f9b88
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)