Prepare the repository for handoff

[dv00d00]

Summary

• restore a supported .NET 10 default for the full solution while keeping the .NET 11 platform-async lane opt-in
• remove the redundant net10 Pipelines package reference and collapse the test corpus copy list to stable globs
• add a zero-test guard, a build/test workflow, and an explicit map of the three related AngleSharp upstream pull requests

No public library behavior changes.

Validation

• dotnet restore AngleSharp.ReadOnlyDom.slnx --force --no-cache
• dotnet build AngleSharp.ReadOnlyDom.slnx -c Release --no-restore -m:1 (0 warnings, 0 errors)
• dotnet test tests/AngleSharp.ReadOnlyDom.Tests/AngleSharp.ReadOnlyDom.Tests.csproj -c Release -f net10.0 --no-build --no-restore -- --minimum-expected-tests 1 --progress off (179562 passed)
• Hacker News sample smoke test: HTTP 200 from the default net10 build