signed-receipts/v1: signed task receipts for A2A agents (draft 0.3), with conformance vectors and runners

Draft 0.3 of signed-receipts/v1, an extension to the Agent2Agent (A2A) protocol: a receipt object an agent can attach to a completed task, canonicalised with RFC 8785 (JCS) and signed with Ed25519 under a did:web key, so that a third party can check offline what the issuer claimed and when. What changed from draft 0.2. (1) An erratum: draft 0.2's change notes said that characters outside the Basic Multilingual Plane are escaped as surrogate pairs. RFC 8785 writes them as themselves; draft 0.3 withdraws the sentence and states the full string and number rules. (2) Verification has three normative results, VALID, INVALID and UNVERIFIABLE_KEY. A receipt whose key cannot be resolved is never VALID. (3) signer_public_key and register are listed as members of the receipt. (4) The conformance vectors are normative for canonicalisation and verification. Files. SPEC-draft-0.3.md is the document of record (sha256 91520cda9998c613377f1cdae2ae7a8dc5464cf3eab1f3f6a5ef269e0d938f58). SPEC-draft-0.2.md is the superseded draft, byte for byte (sha256 f5a7400b1963473718156d14e70df6c640ee12881e6c56dc5ecbfac0e9e43efa). vectors.json holds 17 core Ed25519 cases and 4 optional ML-DSA-65 interop cases, the latter copied unmodified from @fractalai/pqc-agent-receipts-conformance 0.3.1 (Apache-2.0). run.mjs (Node 20 or later, no dependencies) and run.py (Python with cryptography) check a results file against the vectors. signed-receipts-v1-draft-0.3-kit.zip holds the whole kit in its served layout: both drafts, the reference implementation interceptor.py and its checks test_interceptor.py, and conformance/ with the vectors, both runners, reference-results.json (the reference verifier's results; every case matches), example-fail-results.json (the verifier as it was before 2026-09-28; both runners report FAIL on it) and gen_vectors.py (regenerates vectors.json byte for byte). Two entries in the public corrections ledger, C-2026-0928-01 and C-2026-0928-02 (https://councilof.ai/api/corrections), describe the faults this draft corrects. IETF SCITT architecture issue 462 cites the pre-correction verifier. A PASS from the runners means that an implementation's results equal the expected results. It is not a mark of conformity, approval or quality, and nobody issues one. A receipt is evidence of what its issuer claimed and when; it does not show that the claim is true.