xunit-v3-mtp-test-stack

Guidance for xUnit v3 on Microsoft.Testing.Platform in .NET repositories. Use when setting the repo test runner, choosing xUnit v3 MTP package variants, writing `dotnet test --project` or `--solution` commands, using xUnit filter switches, configuring MTP-native coverage, designing assembly fixtures, or troubleshooting zero-test and unsupported-switch failures.