alice writes M1 on node-1
bob writes M2 on node-2
  M1=cef01b1d-cd52-4347-8e78-7102417ce1cc M2=e14d6da4-fdf3-4392-8035-ecb024c12bd0
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)