alice writes M1 on node-1
bob writes M2 on node-2
  M1=405a3870-8ad7-4451-9ca1-8d807392d28a M2=8dfa3039-b49b-4bab-930d-79b7a9cb8850
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)