Migrate test framework from xUnit to TUnit

[Copilot]

Replace xUnit with TUnit as the sole test framework, as the existing tests use only basic xUnit primitives with direct TUnit equivalents.

Changes

• DayPhaseIndicator.Tests.csproj — Swap 3 xUnit packages (xunit, xunit.runner.visualstudio, Microsoft.NET.Test.Sdk) for single TUnit 1.37.24. Add OutputType: Exe + TestingPlatformDotnetTestSupport (required by TUnit's source-gen runner model).
• global.json (new) — Opt in to Microsoft.Testing.Platform runner, required for dotnet test on .NET 10 SDK (VSTest no longer supported).
• PhaseLogicTests.cs — Rewrite using TUnit API:

// Before (xUnit)
[Theory]
[InlineData(10, 0, "WORK")]
public void FindCurrentPhase_StandardSchedule(int hour, int minute, string expectedLabel)
{
    Assert.Equal(expectedLabel, phase.Label);
}

// After (TUnit)
[Test]
[Arguments(10, 0, "WORK")]
public async Task FindCurrentPhase_StandardSchedule(int hour, int minute, string expectedLabel)
{
    await Assert.That(phase.Label).IsEqualTo(expectedLabel);
}

All 15 tests pass on both dotnet test (solution-level) and direct executable invocation.