alice writes M1 + M2 + links M1→M2
  M1=6d14f400-6999-4afc-8c75-4c52f4bbe4d5 M2=ac979b77-8f79-44bb-adaf-342b2bfc885b
settle 6s for link fanout
charlie queries /api/v1/links/M1 (forward)
charlie queries /api/v1/links/M2 (reverse)