Document hotspot grouped count semantics

[Widthdom]

Summary

• Documented that hotspots --group-by-name JSON count is the returned name/kind group count.
• Updated help/completion copy and USER_GUIDE EN/JA wording to distinguish grouped count, definition sites, references, and files.
• Strengthened the grouped-hotspots JSON test to pin count separately from reference_count and definition_sites.

Validation

• dotnet run --project tools/CodeIndex.Changelog -- check
• dotnet build CodeIndex.sln /p:EnableAotAnalyzer=false /p:EnableTrimAnalyzer=false
• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj --filter "FullyQualifiedName~QueryCommandRunnerTests.RunHotspots_GroupByNameJson_CountIsNameKindGroupCount|FullyQualifiedName~ConsoleUiTests.PrintCompletions_BashAndZshScopeGroupByNameToHotspots" /p:EnableAotAnalyzer=false /p:EnableTrimAnalyzer=false
• dotnet ./src/CodeIndex/bin/Debug/net8.0/cdidx.dll status --check --json
• dotnet ./src/CodeIndex/bin/Debug/net8.0/cdidx.dll index . --rebuild --yes --json
• Adversarial review: No blocking/actionable issues found.

Note: the same build/test/format commands without analyzer overrides currently fail on existing IL3050 AOT analyzer errors outside this change; see #2796.

Documentation / Changelog

• Updated USER_GUIDE.md.
• Added changelog.d/unreleased/1596.docs.md.

Follow-up Candidates

• None.

Fixes #1596