Add search batch and grouped count modes

[Widthdom]

Summary

• Add search --named-query <name>=<query> support with grouped compact JSON/human output for repeated named searches.
• Add search --group-by file|symbol --count support for grouped count summaries.
• Tighten validation around search submodes so unsupported flag combinations fail explicitly.

Validation

• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj --filter "FullyQualifiedName~Issue3481|FullyQualifiedName~Issue3388|FullyQualifiedName~CliFlagSchemaTests|FullyQualifiedName~ConsoleUi"
• dotnet run --project tools/CodeIndex.Changelog -- check
• dotnet build
• dotnet ./src/CodeIndex/bin/Debug/net8.0/cdidx.dll index . --commits HEAD --json
• dotnet ./src/CodeIndex/bin/Debug/net8.0/cdidx.dll status --check --json
• Codex adversarial review: No blocking/actionable issues found.

Documentation / Changelog

• Updated USER_GUIDE.md with named query and grouped count examples.
• Added changelog fragments:
  • changelog.d/unreleased/3481.added.md
  • changelog.d/unreleased/3388.added.md

Scope Notes

• Search results should identify test fixture matches separately from production code #3450 was skipped because it already had a work-started comment.

Follow-up Candidates

• None.

Fixes #3481
Fixes #3388