alice writes M1 on node-1
bob writes M2 on node-2
  M1=36871c87-7ed5-4776-8bdb-923eb5d3b16e M2=0669635a-92b7-465b-9603-028bdaacf1cd
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)