alice writes M1 on node-1
bob writes M2 on node-2
  M1=4326350a-2c34-4509-9a47-2f9cab41a146 M2=d5158bd6-4ad1-49fc-ab44-093d235e821b
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)