alice writes M1 on node-1
bob writes M2 on node-2
  M1=41bd2010-bdc4-47bd-b912-7990185800c0 M2=32da7804-b110-462c-8da5-96f02094f765
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)