Split extractor plugin registry responsibilities

[Widthdom]

Summary

• Split ExtractorPluginRegistry into focused partial files for discovery, diagnostics, pattern config handling, and plugin assembly loading.
• Kept the existing registry facade and extension behavior intact.
• Added an issue-based bilingual changelog fragment.

Validation

• dotnet build
• dotnet format CodeIndex.sln --verify-no-changes
• dotnet run --project tools/CodeIndex.Changelog -- check
• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj --filter FullyQualifiedName~ExtractorPluginRegistryTests
• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj --filter FullyQualifiedName~ConfiguredPatternYaml
• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj --filter FullyQualifiedName~Run_StatusJsonIncludesExtractorPluginDiagnostics
• dotnet ./src/CodeIndex/bin/Debug/net8.0/cdidx.dll status --check --json

Full dotnet test was started but manually stopped after about 8 minutes with no completion output; targeted validation for the touched extension-boundary behavior passed.

Documentation / Changelog

• Added changelog.d/unreleased/3422.internal.md.
• No user-facing documentation changes were required because this is an internal structure split with preserved behavior.

Review

• Adversarial review result: No blocking/actionable issues found.

Follow-up Candidates

• None.

Fixes #3422