Add oval ROI tool

[yangfei]

Summary

• Add core OvalRoi model with discrete pixel containment.
• Add oval measurement support that samples only pixels inside the oval.
• Add Edit > Rectangle Selection and Edit > Oval Selection tools.
• Render oval ROI overlays and use oval measurements for active oval selections.
• Keep histogram/profile limited to full image or rectangle ROI with explicit unsupported-selection messages.

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 #19