alice writes M1 on node-1
bob writes M2 on node-2
  M1=4193aeeb-527c-45cc-ae1e-abeaca21e9de M2=2dc58839-6055-4bce-ad11-d824221ff4da
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)