alice writes M1 on node-1
bob writes M2 on node-2
  M1=7dc215a5-4712-4e0f-9962-58dff57b1e0e M2=578600f1-1ad9-420b-9b36-9275c2812f83
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)