fast-jwt: Verifier cache accepts expired JWTs without iat.

Summary cacheSet only derives its exp cache deadline inside hasIat src/verifier.js:127-140. JWT iat is optional. For a valid token with exp but no iat, cacheSet substitutes clockTimestamp + clockTolerance + cacheTTL src/verifier.js:142-146. Later, the cache path returns the saved payload before...