alice writes M1 on node-1
bob writes M2 on node-2
  M1=763c257e-83ed-49da-bb30-5570b8527d67 M2=13af0446-8efe-4ef6-9bf1-5d32edd1bda4
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)