Bound index extraction stall handling

[Widthdom]

Summary

• Prevented the symbol extractor from restarting at the same or an earlier offset during same-line pattern rescans.
• Added bounded extraction-stall diagnostics for update and full-scan symbol extraction paths, including stable error code E013_INDEX_EXTRACTION_STALLED.
• Propagated stall failures to the top-level command result and cancel extraction workers when a full-scan stall is detected.

Validation

• dotnet build CodeIndex.sln -c Debug -p:UseSharedCompilation=false
• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj -c Debug -p:UseSharedCompilation=false --filter FullScanExtractionStallPath
• dotnet run --project tools/CodeIndex.Changelog -- check
• git diff --check origin/main..HEAD
• Adversarial review: No blocking/actionable issues found.

Full dotnet test CodeIndex.sln -c Release -p:UseSharedCompilation=false was attempted earlier but did not complete within the bounded wait in this local environment.

Changelog

• changelog.d/unreleased/2665.fixed.md

Notes

The three requested issues share the same extraction-stall root cause, so the implementation is intentionally shared instead of splitting artificial per-issue code paths.

Fixes #2665
Fixes #2680
Fixes #2683