update refs

[SimonCropp]

NEEDS A SQUASH ON MERGE

Update refs: xunit.v3 4.0

Bumps xunit.v3 from 3.2.2 to 4.0.0, plus the fallout from that major bump.

Package updates

Package	From	To
xunit.v3	3.2.2	4.0.0
Roslynator.Analyzers	4.16.0	4.16.1
SDK (global.json)	10.0.100	10.0.400

Removed packages

• xunit.runner.visualstudio — a VSTest adapter. xunit.v3 4.x runs natively on
  Microsoft.Testing.Platform (MTP), so the adapter is dead weight.
• Microsoft.NET.Test.Sdk — VSTest infrastructure, likewise unused under MTP.

Removing Microsoft.NET.Test.Sdk meant losing the OutputType=Exe it was setting
implicitly, and xunit.v3 requires test projects to be executable. Both test projects
now set <OutputType>Exe</OutputType> explicitly.

Opt in to MTP mode of dotnet test

xunit.v3 4.0 pulls in Microsoft.Testing.Platform 2.x, which dropped the VSTest bridge
on the .NET 10 SDK. Without the opt-in, dotnet test fails with:

Testing with VSTest target is no longer supported by Microsoft.Testing.Platform on
.NET 10 SDK and later.

The opt-in goes in global.json:

"test": {
  "runner": "Microsoft.Testing.Platform"
}

Note this is the .NET 10 mechanism — the older TestingPlatformDotnetTestSupport
MSBuild property no longer works, since MTP 2 removed that path on .NET 10.

build.cs needed no change: MTP mode still accepts the solution positionally, so the
existing DotNetTest("./src/Verify.Terminal.slnx", …) call works as-is.

Obsolete parallelization API

CollectionBehaviorAttribute.DisableTestParallelization is obsolete in 4.0, and the
build treats all warnings as errors, so this was a hard build failure. Replaced with
the new attribute in GlobalUsings.cs:

[assembly: Parallelization(Mode = ParallelMode.None)]

Behaviour is unchanged — the integration tests still run serially, which they need to
because Verify keeps global static naming state and these tests hit the real filesystem.

Verification

Full dotnet build.cs (Build → Test → Pack) is green: 54/54 tests passing across
net8.0 and net10.0, package produced, zero warnings.