alice writes M1 + M2 + links M1→M2
  M1=fa16e787-3c1b-465d-9409-8a303482b3c2 M2=51fda7b8-d6da-4a46-b0fc-58299f3aaca6
settle 6s for link fanout
charlie queries /api/v1/links/M1 (forward)
charlie queries /api/v1/links/M2 (reverse)