Move all C# related files into ./csharp folder

[konard]

Summary

This PR reorganizes the repository structure by moving all C# related files into a dedicated ./csharp folder, improving the organization of this multi-language comparison project.

Changes Made

File Structure

• ✅ Created ./csharp directory
• ✅ Moved all C# source files (.cs, .csproj, .sln) to ./csharp
• ✅ Moved C# directories (Doublets/, Model/, SQLite/) to ./csharp
• ✅ Moved C# specific .gitignore to ./csharp
• ✅ Created new root-level .gitignore for multi-language repository

Configuration Updates

• ✅ Updated .github/workflows/CI.yml:
  • Added working-directory: csharp to run builds in the correct location
  • Added path filters to only trigger CI on C# changes
  • Maintains compatibility with existing build commands
• ✅ Fixed Comparisons.SQLiteVSDoublets.sln to remove broken external project reference

Testing

• ✅ Verified local build succeeds: dotnet build -c Release
• ✅ All C# files moved successfully using git mv to preserve history
• ✅ CI configuration updated to work with new structure

Repository Structure

.
├── .github/
│   └── workflows/
│       ├── CI.yml (updated)
│       └── cpp.yml
├── cpp/
│   └── (C++ implementation)
├── csharp/
│   ├── .gitignore
│   ├── Benchmarks.cs
│   ├── Comparisons.SQLiteVSDoublets.csproj
│   ├── Comparisons.SQLiteVSDoublets.sln
│   ├── Doublets/
│   ├── Model/
│   ├── Program.cs
│   ├── SQLite/
│   ├── SizeAfterCreationColumn.cs
│   ├── TestRun.cs
│   └── TestRunResults.cs
├── .gitignore (new)
├── README.md
└── README.ru.md

Benefits

• 📁 Better organization for multi-language comparisons
• 🔧 Easier maintenance with language-specific directories
• 🚀 Clear separation between C# and C++ implementations
• ✨ Improved repository navigation

Fixes #101

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[konard]

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💰 Cost estimation:

• Public pricing estimate: $1.627446 USD
• Calculated by Anthropic: $0.883772 USD
• Difference: $-0.743674 (-45.70%)
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