alice writes M1 on node-1
bob writes M2 on node-2
  M1=522a6286-f5df-46aa-805c-aff36b58aae3 M2=add652cc-6659-42e1-bbc1-fbcaaaca6fe5
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)