alice writes M1 on node-1
bob writes M2 on node-2
  M1=f1949d5c-c4bc-4afc-b492-73a8dc3c4981 M2=d3d7067d-8ee8-4ff2-8ba6-01dcb18b82be
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)