Clean project files and refresh packages

[dv00d00]

Summary

• centralize nullable, implicit usings, and the parked packaging policy in Directory.Build.props
• pin the .NET 10 SDK lane, make nuget.org explicit, remove redundant benchmark properties and framework package references, and load Windows diagnostics only on Windows
• update AngleSharp fallback 1.7.0 to 1.7.1, FsCheck 3.3.4 to 3.4.0, Microsoft.Extensions.ObjectPool and System.IO.Pipelines to 10.0.11, Polyfill 8.9.0 to 11.2.0, and TUnit 1.63.0 to 1.65.38

No runtime library behavior changes.

Validation

• full solution restore succeeded with the repository NuGet.Config
• full Release multi-target build: 0 warnings, 0 errors, including net472
• net10 test suite: 179562 passed
• no direct vulnerable, deprecated, or outdated repository packages remain