alice writes M1 + M2 + links M1→M2
  M1=c5b77fa9-1dd7-4eff-ba76-ea85c790dff5 M2=488f3085-8317-4b0a-a783-2c2f6c1b184e
settle 6s for link fanout
charlie queries /api/v1/links/M1 (forward)
charlie queries /api/v1/links/M2 (reverse)