Fix SQL MERGE column references

[Widthdom]

Summary

• Add SQL MERGE action parsing for UPDATE SET, INSERT (...), VALUES (...), and ON predicates.
• Emit column_reference and join_condition_reference edges so MERGE-driven lineage is visible.
• Add focused SQL reference extractor coverage and a bilingual changelog fragment.

Validation

• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj -c Release -p:UseSharedCompilation=false --filter "FullyQualifiedName~ReferenceExtractorTests"
• dotnet run --project tools/CodeIndex.Changelog -- check
• dotnet build src/CodeIndex/CodeIndex.csproj -p:UseSharedCompilation=false
• dotnet ./src/CodeIndex/bin/Debug/net8.0/cdidx.dll index . --commits 8a89bae1cfab976c0fb25b1273205778551254f1 --json
• codex exec adversarial review: No blocking/actionable issues found.

Full test note:

• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj -c Release -p:UseSharedCompilation=false was also run. It failed outside this change's scope: Rust associated type defaults are already tracked by Rust associated type defaults are not emitted for trait symbols #2603, and a macOS arm64 ILLink AccessViolationException during trimmed publish was filed as macOS arm64 trimmed publish test can crash in ILLink #2606.

Documentation And Changelog

• Added changelog.d/unreleased/2100.fixed.md.
• No README/user guide changes were needed; this preserves the existing SQL reference extraction surface while filling missing MERGE edges.

Follow-Up Candidates

• Rust associated type defaults are not emitted for trait symbols #2603
• macOS arm64 trimmed publish test can crash in ILLink #2606

Fixes #2100