Fix C# generic type argument graph edges

[Widthdom]

Summary

• Add generic_type_argument references for explicit C# generic invocations such as Process<IFoo>(value).
• Include those edges in call graph traversal, ranking, callers, and impact analysis without promoting ordinary type_reference rows.
• Add focused extractor and DbReader coverage plus a changelog fragment.

Fixes #2062

Validation

• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj --filter "FullyQualifiedName~ReferenceExtractorTests.Extract_CSharpGenericInvocation_EmitsGraphTypeArgumentReference|FullyQualifiedName~DbReaderTests.GetCallers_CSharpGenericInvocationTypeArgument_ParticipatesInGraph" -p:UseSharedCompilation=false
• dotnet run --project tools/CodeIndex.Changelog -- check
• dotnet build CodeIndex.sln -p:UseSharedCompilation=false
• dotnet test CodeIndex.sln -p:UseSharedCompilation=false