Fix #351: capture C# explicit-interface event names

[Widthdom]

Summary

• add a dedicated C# explicit-interface event extractor row so event EventHandler IFoo.Evt is indexed as Evt
• add regression coverage for same-line/next-line accessor blocks plus generic and global:: qualifiers
• update the changelog and developer guide for the new explicit-interface event support

Verification

• dotnet build
• dotnet test
• dotnet test --filter "FullyQualifiedName~Extract_CSharp_DetectsExplicitInterfaceEventImpl"
• dotnet test --filter "FullyQualifiedName~CodeIndex.Tests.SymbolExtractorTests"
• dotnet test --filter "FullyQualifiedName~QueryCommandRunnerTests&FullyQualifiedName~Outline"
• local cdidx.dll repro for definition Evt / outline

Closes #351.