Fix full scan extraction after HEAD changes

[Widthdom]

Summary

• Parallelize full-scan extraction when the existing index was stamped from a different Git HEAD.
• Keep post-extraction hook ordering intact by falling back to sequential reference extraction when hooks are configured.
• Add regression coverage for head-change scheduling and hook-aware fallback.

Validation

• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj -c Release --filter "FullyQualifiedName~IndexCommandRunnerTests.Run_FullScanAfterHeadChange" -p:UseSharedCompilation=false
• dotnet build CodeIndex.sln -c Release -p:UseSharedCompilation=false
• dotnet run --project tools/CodeIndex.Changelog -- check
• dotnet ./src/CodeIndex/bin/Release/net8.0/cdidx.dll . --json
• Adversarial review: No blocking/actionable issues found.

Documentation / Changelog

• Added changelog fragment: changelog.d/unreleased/2605.fixed.md

Follow-up Candidates

• None.

Fixes #2605