alice writes M1 on node-1
bob writes M2 on node-2
  M1=6f92bfdf-a8c4-4218-a18f-5fe4a42a1c34 M2=cd9255d4-0f7a-4366-b9cf-c3eb28094c09
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)