alice writes M1 on node-1
bob writes M2 on node-2
  M1=2e0d8a0b-b6dc-491a-ba50-ba109e002639 M2=29d66079-dfd0-49ec-8791-5d8c41080420
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)