- Implement gpgVerify function that creates a temporary keyring to verify
detached signatures against embedded public keys
- Signature verification happens during deserialization after hash
validation but before decompression
- Extract signatureString() as a method on manifest for generating the
canonical signature string (MAGIC-UUID-MULTIHASH)
- Add --require-signature flag to check command to mandate signature from
a specific GPG key ID
- Expose IsSigned() and Signer() methods on Checker for signature status
- Add --sign-key flag and MFER_SIGN_KEY env var to gen and freshen commands
- Sign inner message multihash with GPG detached signature
- Include signer fingerprint and public key in outer wrapper
- Add comprehensive tests with temporary GPG keyring
- Increase test timeout to 10s for GPG key generation
