alice writes M1 + M2 + links M1→M2
  M1=7ce27bbd-1173-4276-8ce2-27583f542843 M2=d7b4b7d0-6c6e-4485-9724-c4da0710e0d5
settle 6s for link fanout
charlie queries /api/v1/links/M1 (forward)
charlie queries /api/v1/links/M2 (reverse)