alice writes M1 + M2 + links M1→M2
  M1=ec00be6f-504b-4ee6-aa73-c7b59e0e53e0 M2=d7fb1ce8-8190-40a1-870e-06b456fc10bd
settle 6s for link fanout
charlie queries /api/v1/links/M1 (forward)
charlie queries /api/v1/links/M2 (reverse)