Fix C# symbol extraction self-index stalls

[Widthdom]

Summary

• Reuse the per-line C# multi-line member candidate while matching symbol patterns, avoiding repeated expensive lookahead on large extractor sources.
• Add a realistic ReferenceExtractor.cs runaway regression guard.
• Add the unreleased changelog fragment for index: full scan stalls extracting SqlReferenceExtractor.cs symbols #2710, Full repository index can time out near the final extractor files #2711, and Full index can stall on current repository after branch refresh #2717.

Fixes #2710
Fixes #2711
Fixes #2717

Related: #2730

Validation

• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj --filter "FullyQualifiedName~SymbolExtractorTests.Extract_CSharp_ReferenceExtractorFixture_CompletesWithinPracticalBudget|FullyQualifiedName~SymbolExtractorTests.Extract_CSharp_InstallScriptFixture_CompletesWithinPracticalBudget" -p:UseSharedCompilation=false
• dotnet ./src/CodeIndex/bin/Debug/net8.0/cdidx.dll . --json
• dotnet ./src/CodeIndex/bin/Debug/net8.0/cdidx.dll status --check --json
• dotnet run --project tools/CodeIndex.Changelog -- check
• Codex adversarial review: no blocking/actionable issues found.

Notes

A full solution test run emitted unrelated TeeTextWriter disposal and HTTP MCP timeout failures, tracked separately in #2730.