alice writes M1 on node-1
bob writes M2 on node-2
  M1=c56e5cd2-d87b-4034-9615-e3f920e70369 M2=4f31452a-2c30-476e-8000-9aea89a87549
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)