Speed up cdidx indexing and add symbols-only mode

[Widthdom]

Summary

• Add cdidx . --symbols-only as a faster first-pass indexing mode that builds chunks, symbols, and issues while deferring reference graph extraction.
• Reduce cdidx . work after branch or HEAD changes by reusing unchanged files before symbol/reference extraction when safe.
• Optimize fresh full scans by batching reference finalization, avoiding unnecessary empty-database cleanup probes, and narrowing reference-line ID lookups to the current insert batch.
• Reuse C# reference extraction line-state scanning to avoid a duplicate pass over source lines.
• Update README, USER_GUIDE, changelog fragments, and regression tests for the new indexing behavior.

Notes

--symbols-only makes search, definition, symbols, and map workflows usable sooner. Reference graph commands remain degraded until a normal cdidx . run completes.

Testing

• dotnet test