alice writes M1 on node-1
bob writes M2 on node-2
  M1=401422bb-fd4c-4dcb-8a16-d72d689884ed M2=286a5fe7-bf36-46ba-94fd-8068698477b4
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)