alice writes M1 on node-1
bob writes M2 on node-2
  M1=d4f621b3-dd6d-4852-901c-9de59f1d9f0e M2=ddd9058b-5ada-45b6-a22e-53c1ee55b564
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)