Fix C# polymorphic impact traversal

[Widthdom]

Summary

• Expand C# impact traversal so concrete implementation queries also match calls recorded against inherited abstract/interface method targets.
• Include implemented interfaces when walking C# containing type inheritance.
• Add regression coverage for interface dispatch, abstract base dispatch, same-name hierarchy false positives, and duplicate short type names.

Validation

• dotnet build
• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj --filter "FullyQualifiedName~DbReaderTests.GetTransitiveCallers_CSharpExact_MapsInterfaceDispatchToConcreteImplementation|FullyQualifiedName~DbReaderTests.GetTransitiveCallers_CSharpExact_FollowsAbstractBaseDispatchToConcreteOverride|FullyQualifiedName~DbReaderTests.GetTransitiveCallers_CSharpExact_DoesNotMixUnrelatedSameNameHierarchies|FullyQualifiedName~DbReaderTests.GetTransitiveCallers_CSharpExact_DoesNotUseBaseListFromDuplicateShortTypeName"
• Adversarial review: two actionable findings found and addressed.

Documentation and Changelog

• Added changelog.d/unreleased/2060.fixed.md.
• No README/guide updates were needed because this fixes existing graph traversal behavior without changing CLI flags or documented command contracts.

Fixes #2060

Follow-up Candidates

• None.