Add basic process filters

[yangfei]

Summary

• Add fixed 3x3 Gaussian blur, median filter, and sharpen algorithms in core processing.
• Add Process menu commands for Gaussian Blur, Median, and Sharpen.
• Add deterministic core tests and App command-state coverage.

Verification

• dotnet build ImageJCsharp.sln
• dotnet test tests/ImageJCsharp.Core.Tests/ImageJCsharp.Core.Tests.csproj
• dotnet test tests/ImageJCsharp.App.Tests/ImageJCsharp.App.Tests.csproj

Closes #22