alice writes M1 on node-1
bob writes M2 on node-2
  M1=88ebac2e-2117-4225-9951-997932e937f2 M2=a7ea7841-1245-4221-a2d1-652d167be6f5
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)