Add brightness and contrast display adjustment

[yangfei]

Summary

• Add display-only min/max adjustment for grayscale rendering.
• Add Image > Brightness/Contrast... and Image > Reset Display.
• Keep processing, measuring, and saving backed by unchanged GrayImage pixels.
• Add bitmap mapping tests that verify display changes do not mutate source pixels.

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 #17