alice writes M1 on node-1
bob writes M2 on node-2
  M1=684cac9e-6358-421a-99e2-e5bb2336a942 M2=5a6a26c9-e909-4be0-9d93-8d0f6d114da7
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)