alice writes M1 on node-1
bob writes M2 on node-2
  M1=81b2f6e6-c678-4d78-9d13-c69587dc9916 M2=c75870bc-d8f2-4efd-9030-65039a972da1
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)