alice writes M1 on node-1
bob writes M2 on node-2
  M1=84c05cd9-f485-4d7d-af86-bd8cc5a4cca9 M2=8cdcb3a3-3bb4-48b8-a736-ae4ede50ece2
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)