How many effects occur in the default trace?
Two effects. The first lost acknowledgement does not remove its effect.
What duplicate-harm value makes verification and retry tie?
Verification costs 1. Retry costs 0.2 plus the probability 0.8 that the effect already happened times the duplicate cost d. They tie when 1=0.2+0.8d, so d=1.
What happens when a stored idempotency key is reused with a different payload?
Validation rejects the conflicting key/payload contract.