alice writes M1 on node-1
bob writes M2 on node-2
  M1=ea1fe68b-5f8d-4fbe-99e2-e38584540eda M2=e837ae37-a87f-4ad1-a8a4-b0c884c8b583
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)