A receipt is the raw evidence attached to every act: the API response, delivery confirmation, diff, or file hash that proves the outcome. Acts without obtainable receipts are flagged as unverified, not assumed fine.
Claimed outcomes without evidence is how trust in autonomous work dies. Heidi renders the proof next to the claim: you see the work and the receipt.
Receipts also power undo, since you can only confidently reverse what you can prove happened, and they make every run auditable end to end.