Run-receipt format spec sufficient for an independent verifier

## Gap The audit bundle ships a spec-driven standalone verifier (`tools/verify_audit_dsse.py` is written to be re-implementable from its docstring alone). The run receipt has exactly one implementation and no normative format document. An operator handing a receipt to a third party today can only point them at our source code. ## What to write A docs page specifying, field by field: - the subject binding (run id, journal head, spine head, audit range) and its canonicalization profile, - the DSSE PAE payload type and signature input, - embedded-JWK vs pinned-key verdict tiers (integrity-only vs provenance), - the exit-code contract (0 verified / 1 malformed / 2 tamper), - a worked example: one valid receipt and one tampered receipt as test vectors. ## Acceptance criteria - A reader can implement a verifier without opening `src/`; a stdlib-only reference script (or second-language snippet) validates both test vectors. - Test vectors live in the repo and are exercised by CI. ## Out of scope Changing the receipt format itself. Questions welcome in this thread.