[MEDIUM] Fix unencoded payload verification state and crit handling

[OskarEichler]

Summary

Corrects two verification gaps in the b64: false payload path: decoding no longer marks all JWT claims as verified, and unsupported critical JOSE extensions are rejected. This addresses the highest-priority open verification-state finding in #723 and enforces RFC 7515 section 4.1.11.

Problem

unverified_payload invokes decode_payload. For unencoded payloads, that method called verify_claims!(crit: ['b64']), which set the shared @claims_verified flag after checking only the protected-header requirement.

After a valid signature check, this sequence allowed payload to return an expired payload without an explicit claim verification call:

token.verify_signature!(algorithm: 'HS256', key: key)
token.unverified_payload
token.payload

The normal JWT.decode path is not affected.

The same internal path also accepted tokens whose crit array contained unknown extensions. RFC 7515 requires implementations to reject a JWS when any listed critical extension is not understood and supported.

Fix

Invoke the Claims::Crit verifier directly through the existing claims context. This preserves the internal b64 header check without changing the general claims-verification state. A backward-compatible strict option rejects any critical extensions outside the explicitly supported set for this internal path.

Verification

• Focused reproduction on 3.2.0 before: payload_after_unverified=BYPASS
• Focused reproduction after: payload access raises JWT::DecodeError until explicit claim verification
• Expired claim remains rejected by explicit verify_claims!
• Focused unknown-extension reproduction before: token accepted
• Focused unknown-extension reproduction after: JWT::InvalidCritError
• bundle exec rspec on Ruby 4.0.6: 557 examples, 0 failures, 12 expected pending
• bundle exec rspec on Ruby 3.2.11: 557 examples, 0 failures, 12 expected pending
• bundle exec rubocop lib/jwt/claims/crit.rb lib/jwt/encoded_token.rb: no offenses
• gem build ruby-jwt.gemspec: successful
• Ruby 4.0.6, OpenSSL 3.6.0/3.6.3

No test files are included in this focused patch. The behavior was verified with an external reproduction plus the complete existing suite.

Compatibility

No public API or successful verification behavior changes. No breaking change intended.

[anakinj]

There are no specs to ensure the behavior of the strict flag or the changed behavior of the unencoded_payload