alice writes M1 on node-1
bob writes M2 on node-2
  M1=f82d6d02-944b-496d-a8a8-c56d71326bad M2=8976d396-8c53-42eb-9231-226ba531f453
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)