alice writes M1 + M2 + links M1→M2
  M1=4bc6c943-d478-4d3f-ad8b-773e9d003606 M2=e7181295-6c79-4f64-88ab-26d3c2fe0b11
settle 6s for link fanout
charlie queries /api/v1/links/M1 (forward)
charlie queries /api/v1/links/M2 (reverse)