alice writes M1 on node-1
bob writes M2 on node-2
  M1=a5cdc92d-3061-4b71-b74d-cceb57cb885d M2=8943c7bf-786d-46d6-925f-e80b6079a531
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)