alice writes M1 on node-1
bob writes M2 on node-2
  M1=e8e1bcff-b61e-410d-a316-36be58a6793e M2=0a18ad34-f258-405c-9161-c525d0956cce
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)