What changed

• Reworked SelectionItemContentTests.Invalidated_EmitsOnAnimationChange to use FakeTimeProvider instead of real time.
• Subscribes to Invalidated, advances fake time by spinner interval, and asserts deterministic invalidation counts.
• Removed wall-clock WaitAsync timeout dependency.

Why

• CI failed on macOS due to nondeterministic timer scheduling for the real-time spinner test.
• This makes the test deterministic and aligned with repository testing guidelines.

Verification

• dotnet test tests/Termina.Tests/Termina.Tests.csproj -c Release --filter FullyQualifiedName~Termina.Tests.Layout.SelectionItemContentTests.Invalidated_EmitsOnAnimationChange
• dotnet test tests/Termina.Tests/Termina.Tests.csproj -c Release --filter FullyQualifiedName~Termina.Tests.Layout.SelectionItemContentTests