alice writes M1 on node-1
bob writes M2 on node-2
  M1=32b7ba78-342b-4e85-a292-5c2aef8a6290 M2=4d0bca45-2cd7-4a4b-bce7-6c747ad5cbeb
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)