alice writes M1 on node-1
bob writes M2 on node-2
  M1=72c05810-6717-4679-ab0d-594fcf758817 M2=5749d376-9fc7-49cd-9f23-928fbd2c9e21
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)