Authenticate Execution Passports without coupling the core

[aantenore]

What changed

• compose complete Execution Passport artifact verification with a provider-neutral authenticated-envelope port
• add an optional Sigstore subpath that requires one DSSE signature, exact canonical payload bytes and literal issuer-plus-SAN authorization
• keep public Sigstore CT/Rekor thresholds separate from caller-owned enterprise verifier policy
• fail closed on malformed verifier results and unsigned downgrade attempts
• document the trust boundary, Node compatibility and non-claims

Why

An unsigned Passport can prove that a complete artifact set is internally consistent, but not who produced it. This adds an explicit authenticated-required path while keeping the default package provider-neutral and free of mandatory Sigstore dependencies.

Verification

• npm run release:check
• 110/110 tests
• 92.47% statement coverage
• production audit: 0 vulnerabilities
• publint and package type checks
• installed-package smoke with and without optional Sigstore peers
• all-ref identity and forbidden-string audit