alice writes M1 on node-1
bob writes M2 on node-2
  M1=a1142989-bd65-49e6-a19b-f720f097e8df M2=e8b55021-2e77-4f03-919e-abbb9e7d5976
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)