⚡ Bolt: Optimize GetPerAppWritableRoot string sanitization

[manupawickramasinghe]

💡 What

Optimized application name path sanitization in KernelMemoryCompatExports.GetPerAppWritableRoot() by replacing Path.GetInvalidFileNameChars().Contains(...) and LINQ .Select().ToArray() with a static SIMD-accelerated SearchValues<char> instance and string.Create.

🎯 Subsystem & Bottleneck

• Subsystem: SharpEmu.Libs Kernel Exports (KernelMemoryCompatExports.cs)
• Bottleneck: GetPerAppWritableRoot() was allocating a new char[] array on every invocation via Path.GetInvalidFileNameChars(), followed by an $O(N \times K)$ LINQ Select projection that evaluated invalidChars.Contains(ch) and allocated a temporary array before creating a new string.

📜 Git & Contributor Context

• GetPerAppWritableRoot() is invoked during kernel initialization and directory resolving for writable app directories (download0, etc.).

📊 Measured Performance Impact

• Algorithm Complexity: Reduced character lookup complexity from $O(N \times K)$ (where $K$ is the length of GetInvalidFileNameChars()) to $O(1)$ SIMD-vectorized character scans via SearchValues<char>.
• Zero-Allocation Fast Path: When no invalid characters are present, no extra string allocation or buffer manipulation occurs.
• Single Allocation Slow Path: When invalid characters are present, string.Create allocates only the target string buffer once.

🔬 Verification Checklist

• Solution builds cleanly with dotnet build SharpEmu.slnx.
• All unit tests pass via dotnet test SharpEmu.slnx.
• Formatting verified with dotnet format SharpEmu.slnx --verify-no-changes --include src/SharpEmu.Libs/Kernel/KernelMemoryCompatExports.cs tests/SharpEmu.Libs.Tests/Kernel/KernelMemoryCompatExportsTests.cs.

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PR created automatically by Jules for task 16479707384063025503 started by @manupawickramasinghe

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📦 Build artifacts — 4ca2c3d

Platform	Download
Windows (win-x64)	sharpemu-win-x64-d417c21
Linux (linux-x64)	sharpemu-linux-x64-d417c21
macOS (osx-x64)	sharpemu-osx-x64-d417c21

From build run #140. Downloads require a GitHub login and expire after 90 days.