alice writes M1 on node-1
bob writes M2 on node-2
  M1=549d70b8-88bd-47b4-a5cc-e6707e8e81b0 M2=d2cb4874-bd9a-4f75-a872-255c30616b45
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)