alice writes M1 on node-1
bob writes M2 on node-2
  M1=35878e6d-3669-4b76-895d-d799d7991afe M2=257cfbdd-03af-43be-bcc5-70f45c8f2629
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)