alice writes M1 on node-1
bob writes M2 on node-2
  M1=7a5bd853-7547-413d-8d1f-0882c31cc4e0 M2=5d622bd8-bca9-4075-ac31-6bd680435271
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)