alice writes M1 on node-1
bob writes M2 on node-2
  M1=81b34785-743b-4425-b2a6-4a378e1bc72e M2=4d9fea0f-369b-43f8-acc5-1a274ac078dc
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)