alice writes M1 on node-1
bob writes M2 on node-2
  M1=d480b8eb-3b72-4955-9b33-c7cb9b0f6268 M2=ca67e1f5-8f29-408b-b898-41e7fee2de82
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)