Fix extraction hot-path regressions

[Widthdom]

Summary

• Add CI allocation-budget tests for C# symbol and reference extraction hot paths.
• Avoid C# property-header fallback rescanning method bodies and local var declarations during symbol extraction.

Validation

• dotnet test --filter "Extract_CsharpManyMethods_DoesNotRescanMethodBodiesAsFieldCandidates|Extract_CSharp_MultilineGenericMethodOverloadsKeepDistinctSignatures|SymbolExtraction_CsharpHotPath_StaysWithinAllocationBudget|ReferenceExtraction_CsharpHotPath_StaysWithinAllocationBudget"
• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj --filter "FullyQualifiedName~SymbolExtractorTests|FullyQualifiedName~PerformanceTests" (run by Codex adversarial review)
• dotnet run --project tools/CodeIndex.Changelog -- check
• dotnet build
• dotnet ./src/CodeIndex/bin/Debug/net8.0/cdidx.dll index . --commits 03c1e7fe30a0232bf23b761d8532dd203cee53c4 --json

Documentation and Changelog

• Added changelog.d/unreleased/1667.internal.md.
• Added changelog.d/unreleased/2740.fixed.md.

Review

• Codex adversarial review: No blocking/actionable issues found.

Follow-up Candidates

• None.

Fixes #1667
Fixes #2740