alice writes M1 on node-1
bob writes M2 on node-2
  M1=8eb9615f-801a-48d5-8e32-f489274d51ef M2=7470356e-0cf7-4490-9466-009e9e0e4de8
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)