alice writes M1 on node-1
bob writes M2 on node-2
  M1=3d98c11f-716c-4b63-9852-937aaed4c558 M2=75ba045d-a454-42e8-98e5-8aa02832fb3b
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)