alice writes M1 on node-1
bob writes M2 on node-2
  M1=85c9603e-9f1b-4256-a7bf-9b4df358ed96 M2=bc7cd93b-bc1f-44b9-b136-5e5aa7ba6043
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)