unsupported_signature_alg· code· HTTP 422· scope bothProof / event signature `alg` is not in the conformance signature-algorithm allowlist (artifacts/registry/signature-alg-registry.json) on a critical field. One of the four algorithm-agility fail-closed errors; dual-registered as a service code and a reason_code (see unsupported_digest_algorithm). See zh/conformance/encoding.md §6.1.