alice writes M1 on node-1
bob writes M2 on node-2
  M1=c850e027-2b6d-421d-856d-d562b27fb3cb M2=7a318f74-1dad-4523-8ce1-c66b38dc774b
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)