alice writes M1 on node-1
bob writes M2 on node-2
  M1=12203381-c106-428d-96f9-f2d1a65b36d6 M2=9ba90617-7ae2-4f16-bf01-c97a1c7e6853
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)