alice writes M1 on node-1
bob writes M2 on node-2
  M1=5195df61-fa78-4edf-a62e-5ae00622a19c M2=0ff6bc01-d26a-47ae-ad77-4594a073ffab
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)