alice writes M1 on node-1
bob writes M2 on node-2
  M1=e30f97a7-7343-4a84-a6dc-747d00e9fef0 M2=db3f8fa7-34f2-4928-a5c4-656f6b5ea934
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)