chore: migrate tests to xUnit.net v3, cover both shipped TFMs, repo maintenance

[StuartMeeks]

xunit → 4.0.0

There is no xunit 4.0.0 — the xunit package tops out at 2.9.3. Version 4.0.0 is xunit.v3, so this is a v2 → v3 migration:

• xunit 2.9.3 → xunit.v3 4.0.0; v3 test projects are self-executing console apps, hence OutputType=Exe.
• v3 4.0.0 runs on Microsoft.Testing.Platform, and MTP dropped VSTest-bridge support on the .NET 10 SDK. dotnet test opts into the MTP runner via a root global.json; TestingPlatformDotnetTestSupport is the old path and now hard-errors.
• Dropped Microsoft.NET.Test.Sdk and xunit.runner.visualstudio — both VSTest-only. (xunit.runner.visualstudio 4.0.0 is still the v2 adapter, not a v3 upgrade path.)
• One code fix: xUnit2033 in GradientTests.cs, surfaced by v3's analyzers and promoted to an error by TreatWarningsAsErrors.

Both shipped TFMs are now tested

The library ships lib/net8.0 and lib/net10.0, but the suite ran on net10.0 only — lib/net8.0 was compiled, never executed. The test project now multi-targets net8.0;net10.0, so the 40 tests run once per TFM against their own dependency graph (80 executions). Confirmed the net8.0 leg binds to the net8.0 asset, not a re-resolved net10 one:

runtimeTarget: .NETCoreApp,Version=v8.0
  NextIteration.SpectreConsole.Splash/0.3.0   ← TFM attribute: .NETCoreApp,Version=v8.0

CI needed the matching runtime — this was not optional. Building an older TFM needs only reference assemblies, but running it needs that runtime installed, so the previous 10.0.x-only setup failed the net8.0 leg outright:

Error output: You must install or update .NET to run this application.
Framework: 'Microsoft.NETCore.App', version '8.0.0' (x64)
Test run summary: Failed!   error: 1

The build job now installs 8.0.x alongside 10.0.x. The build still uses the newest SDK, as global.json carries no sdk pin. The publish job stays on 10.0.x — it executes no test code.

⚠️ Contributor impact: running the full suite locally now requires the .NET 8 runtime alongside the .NET 10 SDK (sudo apt-get install -y dotnet-runtime-8.0). Otherwise use dotnet test --framework net10.0.

Bugs fixed along the way

• PackageOutputPath broke non-Windows builds. With GeneratePackageOnBuild=true, the hardcoded C:\nuget-local\ made every Linux/macOS/CI build write into a literal directory named C:\nuget-local\ under src/. Reproduced, then guarded the property to Windows so the local dev feed still works there.
• The publish job could not check out the repo. Its permissions block listed only id-token: write, and GitHub sets every unlisted scope to none — leaving actions/checkout without contents: read. Worth checking whether the v0.3.0 tag publish actually succeeded.

Cleanup

• Removed Microsoft.SourceLink.GitHub — the .NET SDK has bundled SourceLink since .NET 8. Verified rather than assumed: packed both ways and diffed. Identical .nuspec repository metadata, and .snupkg PDBs still carry raw.githubusercontent source URLs, so consumer step-through debugging is unaffected.
• Bumped actions/checkout v6→v7, setup-dotnet v5→v6, upload-artifact v6→v7, download-artifact v7→v8. Read each release note first — all routine dependency/runtime updates, no input changes affecting this workflow. Dropped the obsolete FORCE_JAVASCRIPT_ACTIONS_TO_NODE24 transitional flag.
• Deduped Nullable/Authors already set by Directory.Build.props.

Verification

Full restore → build → test → pack in Release, matching CI:

build    0 Warning(s)  0 Error(s)
test     net8.0|x64  passed (2s 041ms)
         net10.0|x64 passed (1s 823ms)
         Passed!  total: 80  failed: 0  succeeded: 80  skipped: 0
pack     lib/net8.0 + lib/net10.0, repository metadata intact

No library code, public API, or shipped package contents changed.

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