Implement native library version using NativeAOT

[konard]

📋 Summary

This pull request implements a native library version of Platform.Data.Doublets using .NET NativeAOT (the modern successor to CoreRT) as requested in issue #40.

✅ What's Implemented

• New NativeAOT Project: csharp/Platform.Data.Doublets.NativeLibrary/

  • Modern NativeAOT configuration replacing deprecated CoreRT
  • C-style FFI exports with [UnmanagedCallersOnly] attributes
  • Thread-safe handle-based API for cross-language compatibility

• Complete FFI Interface: Matches existing c/ffi.h specification

  • UInt64UnitedMemoryLinks_New/Drop - Instance management
  • UInt64UnitedMemoryLinks_Create/Count/Update/Delete - CRUD operations
  • GetLibraryVersion - Version information
  • All exported symbols verified in generated native library

• Cross-Platform Native Compilation:

  • Linux: Platform.Data.Doublets.NativeLibrary.so (2.7MB)
  • Windows: Platform.Data.Doublets.NativeLibrary.dll
  • macOS: Platform.Data.Doublets.NativeLibrary.dylib

• Build Infrastructure:

  • build-native-library.sh - Multi-platform build script
  • test-native-build.sh - Build verification script
  • Updated solution file with new project

• Documentation & Examples:

  • Comprehensive README with usage instructions
  • C example program demonstrating FFI usage
  • Makefile for building and testing C integration

🔧 Technical Details

NativeAOT vs CoreRT Benefits:

• Uses modern .NET 8 NativeAOT instead of deprecated CoreRT
• Better performance with latest AOT optimizations
• Self-contained deployment (no .NET runtime dependency)
• Smaller footprint compared to managed deployment
• Full compatibility with existing FFI consumers

Architecture:

• Handle-based object management (avoids direct memory management across FFI)
• Thread-safe operations with proper locking
• Graceful error handling (returns 0/null on failure)
• Memory-efficient native compilation

🧪 Verification

✅ Build Success: Native compilation completes without errors
✅ Symbol Export: All required C functions properly exported
✅ File Generation: 2.7MB native shared library created
✅ FFI Compatibility: Matches existing ffi.h interface signatures
✅ Cross-Platform: Supports Linux, Windows, and macOS targets

📁 Files Added

csharp/Platform.Data.Doublets.NativeLibrary/
├── Platform.Data.Doublets.NativeLibrary.csproj  # NativeAOT project config
├── DoubletsNativeLibrary.cs                      # FFI implementation  
└── README.md                                     # Documentation

csharp/
├── build-native-library.sh                      # Build script
└── test-native-build.sh                         # Test script

examples/c-native-library-test/
├── test.c                                        # C usage example
├── Makefile                                      # Build configuration  
└── README.md                                     # Example documentation

🎯 Closes

Fixes #40

This provides the modern NativeAOT solution requested in the issue, offering better performance and deployment characteristics than the original CoreRT approach.

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🤖 Generated with Claude Code