Restore .NET Framework 4.8 compatibility

[xincheng213618]

Summary

• Revert the complete change set introduced by Fix SkiaDrawingHelper after RenderToGraphics removal #37 and restore the last known-good Update net3.5->net4.8 && Add NET8.0 WPFdemo #32 implementation.
• Restore ST.Library.UI and WinNodeEditorDemo to .NET Framework 4.8 and their original WinForms/GDI public contracts.
• Restore the existing WpfNodeEdittorDemo sources so they match the original library contract.
• Add the four missing solution build mappings so Any CPU, x86, and Mixed Platforms all build the complete intended project set.

Why a full revert

#37 converted the existing WinForms library from .NET Framework 4.8 to .NET 10 and changed public drawing/control contracts from GDI/WinForms types to Skia types. Its final merge also left the solution unbuildable (including malformed code in STNodeEditor.cs and a framework mismatch with the existing WPF demo).

The commits in #37/#38 form one interdependent rendering migration rather than independent fixes. Reverting the merge as a unit avoids keeping partial API changes and returns the original projects to the tested #32 baseline. The new native WPF implementation remains isolated in #40.

Solution configuration

• Map ST.Library.UI into Debug/Release x86 builds using its Any CPU project configuration.
• Map WinNodeEditorDemo into Debug/Release Any CPU builds using its x86 project configuration.
• Existing Mixed Platforms mappings are preserved.

Validation

• Visual Studio MSBuild restore: 0 errors.
• Rebuild succeeds with 0 errors for all six combinations:
  • Debug / Any CPU
  • Debug / x86
  • Debug / Mixed Platforms
  • Release / Any CPU
  • Release / x86
  • Release / Mixed Platforms
• Debug and Release WinForms demos start, respond, close normally, and exit with code 0.
• Debug and Release legacy WPF demos start, respond, close normally, and exit with code 0.
• Apart from the four solution mapping lines, the resulting source tree matches the Update net3.5->net4.8 && Add NET8.0 WPFdemo #32 merge exactly.