alice writes M1 on node-1
bob writes M2 on node-2
  M1=1befe37d-85d2-4a9c-b068-4f6ba3a5fdcd M2=c2d6a066-18f7-438f-9c58-b71216bbad6a
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)