alice writes M1 on node-1
bob writes M2 on node-2
  M1=6f7988ee-9acd-4be2-bdbb-75084deb63fb M2=b69ac4f5-8c26-420d-be72-bc30af5cf717
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)