Infra: Add criterion benchmarks and cargo-fuzz testing

[hahwul]

Summary

Two testing infrastructure additions:

1. Benchmarks (criterion) - Performance regression testing for crack/brute force
2. Fuzz testing (cargo-fuzz) - Robustness testing for JWT/JWE parsing, especially decode_jwe

Acceptance Criteria

• Benchmark suite covering: encode, decode, verify, crack (dict + brute)
• Fuzz targets for: JWT decode, JWE decode, header parsing
• CI integration for benchmarks (compare against baseline)