Fix reference kind histograms

[Widthdom]

Summary

• Preserve raw per-kind reference histograms for grouped callers / callees rows.
• Render human caller/callee output with count-aware kind labels, while keeping the existing scalar and distinct-kind fields.
• Add regression coverage for mixed-kind caller/callee JSON and human output.

Validation

• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj --filter "FullyQualifiedName~DbReaderTests.GetCallers_ExposesDistinctReferenceKindsForMixedGroups|FullyQualifiedName~QueryCommandRunnerTests.RunCallers" -p:UseSharedCompilation=false
• dotnet run --project tools/CodeIndex.Changelog -- check
• dotnet build CodeIndex.sln -c Release -p:UseSharedCompilation=false
• dotnet test CodeIndex.sln -c Release -p:UseSharedCompilation=false
• Adversarial review: No blocking/actionable issues found.

Documentation and changelog

• Added changelog.d/unreleased/1777.fixed.md.
• No AGENTS.md, CLAUDE.md, or AGENT_GUIDE.md changes.

Follow-up candidates

• None.

Fixes #1777