🛡️ Sentinel: [CRITICAL] Fix prototype pollution vulnerability by removing native Object.assign fallback

[ericbfriday]

🚨 Severity: CRITICAL
💡 Vulnerability: The parser uses a custom `assign` function to prevent prototype pollution by blocking dangerous keys (`proto`, `constructor`, `prototype`). However, it has a fallback to the native `Object.assign` which bypasses these checks.
🎯 Impact: An attacker can supply a malicious configuration object during parsing, leading to prototype pollution which could result in Denial of Service or Remote Code Execution depending on the environment.
🔧 Fix: Removed the fallback to the native `Object.assign` so the custom, safe `assign` implementation is always used.
✅ Verification: Ran `npm test` and `npm run bench:luast` successfully.

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PR created automatically by Jules for task 10694187440451239374 started by @ericbfriday

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