Target .NET 10 for HtmlRenderer.Core and HtmlRenderer.WinForms

[kushanp]

Summary

Modernizes HtmlRenderer.Core and HtmlRenderer.WinForms for .NET 10, with related safety and API fixes.

Targeting

• Core: net10.0 (was netstandard2.0;net8.0)
• WinForms: net10.0-windows with UseWindowsForms + Nullable
• Package bump: System.Resources.Extensions → 10.0.10

Bug / safety fixes

• 64-bit-safe P/Invoke for Metafile EMF save and cursor handles (IntPtr instead of truncated int)
• Thread-safe ImageDownloader client list + dedicated lock in ImageLoadHandler
• HtmlToolTip dispose unsubscription order + null/handle guards
• Nullable annotations on key public APIs (SetHtml, selection getters, entity Equals)
• Modern WinForms designer Editor attribute assembly names for .NET 10

Breaking change

This drops multi-targeting for Core (netstandard2.0 / net8.0). Consumers on older TFMs need to multi-target or stay on a previous release. Happy to adjust to multi-target (net8.0 + net10.0) if preferred.

Scope

Only Core + WinForms (no WPF / PdfSharp / Demo changes).

Test plan

• dotnet build -c Release HtmlRenderer.csproj (net10.0)
• dotnet build -c Release HtmlRenderer.WinForms.csproj (net10.0-windows)
• Load assemblies and instantiate HtmlPanel / HtmlContainer.SetHtml(null)

Notes

Rebased onto current master (includes upstream dependency + GHA updates).
A follow-up PR will add HtmlPanel zoom/pan as a separate feature.