Mark impact result row kinds

[Widthdom]

Summary

• Add explicit result_kind markers to impact output: graph for graph caller rows and file_heuristic for heuristic file dependency hints.
• Propagate the marker through CLI/MCP JSON DTOs, golden output, documentation, and targeted regression tests.

Validation

• dotnet build CodeIndex.sln
• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj --no-build --filter "FullyQualifiedName~DbReaderTests.AnalyzeImpact_NotTruncated_LeavesTruncatedReasonNull|FullyQualifiedName~DbReaderTests.AnalyzeImpact_ClassSymbolReturnsHeuristicFileDependencyHints|FullyQualifiedName~QueryCommandRunnerTests.RunImpact_UnicodeTypeEvidenceStillReturnsHeuristicHints|FullyQualifiedName~JsonOutputSnapshotTests"
• dotnet test CodeIndex.sln --no-build (net8.0 full suite passed; net9.0 full suite was rerun via dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj --framework net9.0 --no-build and passed)
• dotnet run --project tools/CodeIndex.Changelog -- check
• Codex adversarial review: No blocking/actionable issues found.

Documentation and Changelog

• Updated DEVELOPER_GUIDE.md.
• Added changelog fragment changelog.d/unreleased/1779.fixed.md.

Follow-up

• cdidx index can self-interrupt during full rebuild and leave the local index empty #2642

Fixes #1779