Add xUnit test infrastructure and deterministic T01–T08 coverage while preserving IL harness behavior

[Copilot]

This PR introduces xUnit-based unit test infrastructure across the core test projects and adds deterministic coverage for the contracts/components delivered in T01–T08. It keeps the existing IL fixture harness as the integration validation path, ensuring static inspection does not execute fixture methods.

• Test infrastructure (xUnit + discoverability)

  • Added Microsoft.NET.Test.Sdk, xunit, xunit.runner.visualstudio, and coverlet.collector to:
    • ClrLens.Tests.Unit
    • ClrLens.Tests.IL
    • ClrLens.Tests.Optimization
    • ClrLens.Tests.Regression
  • Marked these as test projects for dotnet test discovery.
  • Kept ClrLens.Tests.Benchmarks separate from unit-test runner setup.

• Unit test suite for T01–T08 (deterministic / order-independent)

  • Added focused test files under tests/ClrLens.Tests.Unit for:
    • solution/module boundaries (T01),
    • domain contracts + validation + serialization (T02),
    • IR modeling and CIL→IR mapping (T06),
    • CFG/dominance/loops (T07),
    • call graph + unknown effects (T08).
  • Covers positive and negative paths, including localized diagnostic message expectations for contract/stack/branch/CFG/call-graph failure modes.

• IL and fixture-focused tests

  • Added tests/ClrLens.Tests.IL/* for fixture manifest, assembly ingestion (valid/invalid), CIL decode, stack validation, and exception regions (T03–T05).
  • Added reusable test support helpers for fixture metadata decoding and deterministic path/root resolution.

• Harness preservation and integration alignment

  • Refactored tests/ClrLens.Tests.IL/Program.cs into reusable harness logic (FixtureHarness.RunAsync) and wrapped it with xUnit integration coverage.
  • Explicitly verifies harness-only static analysis flow (no fixture method invocation side effects).

• Regression scaffolding for report compatibility

  • Added FixtureRegressionTests and ReportCompatibilityTests in ClrLens.Tests.Regression to lock schema/serialization/validation behavior and support future optimization-phase test expansion (T16–T18).

<ItemGroup>
  <PackageReference Include="Microsoft.NET.Test.Sdk" Version="17.11.1" />
  <PackageReference Include="xunit" Version="2.9.2" />
  <PackageReference Include="xunit.runner.visualstudio" Version="2.8.2" PrivateAssets="all" />
  <PackageReference Include="coverlet.collector" Version="6.0.2" PrivateAssets="all" />
</ItemGroup>