alice writes M1 on node-1
bob writes M2 on node-2
  M1=044f8088-0b5e-48b2-b762-e9d40e9b4f30 M2=3b70af07-366f-4bbf-9ea7-16b4f1531da9
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)