alice writes M1 on node-1
bob writes M2 on node-2
  M1=8c72c2ca-ad37-41e1-828f-9c5a5dbe1b6e M2=01a71a61-4281-4fc1-9ccc-e4f5f88fa72c
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)