Fix C# phantom symbol extraction suffix-guard regressions

[Widthdom]

Summary

• extend the C# type-body gate so field-like function rows for const / static readonly do not leak method-local symbols
• keep the phantom qualified call-site filter for cases like elapsed < TimeSpan.FromSeconds(...) while preserving legal return types such as @new, await, and yield
• add focused regressions plus changelog / developer guide updates in both English and Japanese

Test plan

• dotnet test --filter "Extract_CSharp_ConstLocalsAndQualifiedCallArguments_DoNotLeakPhantomFunctions|Extract_CSharp_VerbatimReturnTypeIdentifiers_AreNotRejectedBySuffixGuard|Extract_CSharp_ContextualKeywordReturnTypeIdentifiers_AreNotRejectedBySuffixGuard"
• dotnet test --filter FullyQualifiedName~SymbolExtractorTests
• dotnet test
• dotnet build
• dotnet ./src/CodeIndex/bin/Debug/net8.0/cdidx.dll index . --commits HEAD --json
• local cdidx.dll smoke repros for TimeSpan.FromSeconds(...), public @new Make(), and public await MakeAwait() / public yield MakeYield()

Closes #496.
Closes #782.
Closes #785.