alice writes M1 on node-1
bob writes M2 on node-2
  M1=1e2715b5-4cfd-4ddd-b2a1-42a68859e321 M2=14f719d7-b5c8-4b84-b2ac-fcb902a48786
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)