alice writes M1 on node-1
bob writes M2 on node-2
  M1=8b49a02d-c463-4fd0-a4f9-393858cd0eb6 M2=44eff3aa-be7b-4f51-864f-09f2e141f635
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)