Fix Go generic symbol extraction

[Widthdom]

Summary

• Index Go embedded generic struct types, while avoiding ordinary named fields.
• Skip Go blank identifiers in var/const declarations.
• Preserve type-parameterized Go interface method signatures.
• Surface Go build directives as annotations and classify import "C" as cgo.

Validation

• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj --filter "FullyQualifiedName~SymbolExtractorTests.Extract_Go" -p:UseSharedCompilation=false
• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj --filter SymbolExtractorTests
• dotnet build CodeIndex.sln -c Release -p:UseSharedCompilation=false
• dotnet test CodeIndex.sln -c Release -p:UseSharedCompilation=false
• dotnet ./src/CodeIndex/bin/Debug/net8.0/cdidx.dll status --check --json
• Adversarial review: No blocking/actionable issues found.

Documentation and Changelog

• Added bilingual changelog fragments:
  • changelog.d/unreleased/2047.fixed.md
  • changelog.d/unreleased/2048.fixed.md
  • changelog.d/unreleased/2049.fixed.md
  • changelog.d/unreleased/2050.fixed.md
• No AGENT.md, CLAUDE.md, or AGENT_GUIDE.md changes.

Fixes #2047
Fixes #2048
Fixes #2049
Fixes #2050