alice writes M1 on node-1
bob writes M2 on node-2
  M1=e0d1e8c0-6e57-417e-a695-220ef7997627 M2=4822a0ca-1072-4ff8-a59d-34df2056d67f
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)