Add solution and manifest structural extraction

[Widthdom]

Summary

• Add first-class language detection and structural extraction for Visual Studio solution files and Windows application manifests.
• Emit solution project symbols, project-reference graph edges, and app manifest metadata symbols with registered kind taxonomy.
• Update status golden output, user documentation, tests, and changelog fragment for the new structural metadata support.

Validation

• dotnet format CodeIndex.sln --no-restore
• dotnet build CodeIndex.sln -p:UseSharedCompilation=false
• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj --framework net8.0 --no-build --filter "Extract_Solution_IndexesProjectEntries_Issue3662|Extract_Solution_IndexesProjectPathReferences_Issue3662|GetCallers_SolutionProjectReference_ParticipatesInGraph_Issue3662"
• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj --framework net9.0 --no-build --filter "Extract_Solution_IndexesProjectEntries_Issue3662|Extract_Solution_IndexesProjectPathReferences_Issue3662|GetCallers_SolutionProjectReference_ParticipatesInGraph_Issue3662"
• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj --framework net8.0 --no-build
• dotnet run --project tools/CodeIndex.Changelog -- check
• dotnet ./src/CodeIndex/bin/Debug/net8.0/cdidx.dll index . --json
• dotnet ./src/CodeIndex/bin/Debug/net8.0/cdidx.dll status --check --json

Documentation / Changelog

• Updated USER_GUIDE.md for solution and application manifest support.
• Added changelog.d/unreleased/3662.added.md.

Follow-up Candidates

• None.

Fixes #3662