scripts: 🛡️ Sentinel: [CRITICAL] Fix command injection in download.pl

[ManupaKDU]

🚨 Severity: CRITICAL
💡 Vulnerability: Command injection and option injection vulnerabilities exist in scripts/download.pl. Unsanitized $url variables are directly interpolated into shell commands. Option injection exists via cat $sq_rfn_path which could be treated as flags by cat if the filename starts with a hyphen.
🎯 Impact: Attackers could execute arbitrary shell commands if they control the download URL.
🔧 Fix: Used shell_quote() to sanitize the $url parameter passed to aria2c and fallback shell commands. Replaced cat $sq_rfn_path with input redirection cat < $sq_rfn_path to prevent option injection. Also removed the duplicate shell_quote subroutine declaration.
✅ Verification: Ran perl -c scripts/download.pl to verify syntax and make help to ensure build system integration is unbroken.

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PR created automatically by Jules for task 6409084823229499627 started by @manupawickramasinghe

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