alice writes M1 on node-1
bob writes M2 on node-2
  M1=812ea32a-8a04-421a-acb1-593ced8ba301 M2=6c6c20b8-39bf-466c-b562-9d2a2d9438c2
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)