fix: accept cacheTTL: Infinity, keep rejecting NaN

[Copilot]

#640 inadvertently made cacheTTL: Infinity a breaking change by applying the same Number.isFinite guard used for the security fix on clockTimestamp/clockTolerance. Unlike those options, cacheTTL: Infinity is safe — the cache window is always clamped to the token's own exp, so no expired token is accepted.

Changes

• src/verifier.js — relax cacheTTL guard from !Number.isFinite to Number.isNaN; clockTimestamp and clockTolerance keep their isFinite check
• test/verifier.spec.js — remove the assertion that Infinity throws; add a test confirming an expired token is still rejected when cacheTTL: Infinity
• README.md — note that Infinity is a valid value meaning "no cap beyond the token's own expiry"

// Before (#640 behaviour — unintended breaking change)
createVerifier({ key, cacheTTL: Infinity }) // throws

// After — works as pre-#640, expired tokens still rejected
createVerifier({ key, cacheTTL: Infinity }) // ok
createVerifier({ key, cacheTTL: NaN })      // still throws

• Fixes cacheTTL should accept Infinity again (breaking change in #640 was not security-required) #651