🛡️ Sentinel: [CRITICAL] Fix path traversal prefix bypass in IO sandbox

[mleem97]

🚨 Severity: CRITICAL
💡 Vulnerability: Path traversal prefix bypass in GregIoLuaModule.cs ResolveSafe due to String.StartsWith check without a trailing slash. E.g. /mods/modA_secret bypasses a check for /mods/modA.
🎯 Impact: Arbitrary file read/write inside other directories that start with the same prefix.
🔧 Fix: Append Path.DirectorySeparatorChar to dataDirFull before StartsWith check, and allow exact matching to dataDirFull.
✅ Verification: Ran dotnet test tests/gregCore.Tests.csproj --filter "FullyQualifiedName!~CablePatchTests".

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PR created automatically by Jules for task 13127323289353621285 started by @mleem97

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