alice writes M1 + M2 + links M1→M2
  M1=ef422a9f-ac46-4953-b322-fc344db63392 M2=bf95c643-3332-4c1f-ae96-b70dc55f8bbf
settle 6s for link fanout
charlie queries /api/v1/links/M1 (forward)
charlie queries /api/v1/links/M2 (reverse)