Reduce repo map aggregation work

[Widthdom]

Summary

• Refactor repo map section building to aggregate file stats once and reuse shared language/module/file-summary intermediates.
• Add regression coverage for map section ordering and counts after the refactor.

Validation

• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj -c Release --filter GetRepoMap_KeepsSectionOrderingAndCountsAfterAggregateRefactor -p:UseSharedCompilation=false
• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj -c Release --filter GetRepoMap -p:UseSharedCompilation=false
• dotnet build CodeIndex.sln -c Release -p:UseSharedCompilation=false
• dotnet run --project tools/CodeIndex.Changelog -- check
• Codex adversarial review: No blocking/actionable issues found.

Documentation / Changelog

• Added changelog fragment: changelog.d/unreleased/1948.fixed.md
• No docs changes required; repo map output contract is preserved.

Fixes #1948