alice writes M1 on node-1
bob writes M2 on node-2
  M1=7aff3cd9-bdfc-4796-8c4b-ebb341dcc73d M2=2e8229da-8a7c-463b-9f16-b6b4ac4fdfb3
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)