alice writes M1 + M2 + links M1→M2
  M1=3f350c11-4e29-4df7-8d37-c49b06706d19 M2=d9660e7a-bbb8-4c07-9d6e-518a36c21bc9
settle 6s for link fanout
charlie queries /api/v1/links/M1 (forward)
charlie queries /api/v1/links/M2 (reverse)