alice writes M1 on node-1
bob writes M2 on node-2
  M1=d4f68be2-56b5-4954-ab46-3ed33bbb740a M2=798bbf29-8f3f-4b30-81c1-933fdda0585a
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)