alice writes M1 on node-1
bob writes M2 on node-2
  M1=d2284b08-db89-47b0-9c5f-7b001726437e M2=a22c8385-3467-4632-8735-b705879263dc
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)