Migrate CI from AppVeyor to GitHub Actions

[Copilot]

This PR migrates the continuous integration from AppVeyor to GitHub Actions, providing better cross-platform testing and modern CI capabilities.

Changes Made

🔄 CI Migration

• Removed: appveyor.yml configuration
• Added: .github/workflows/ci.yml with comprehensive GitHub Actions workflow
• Updated: README.md badge from AppVeyor to GitHub Actions

🚀 Improved CI Capabilities

The new GitHub Actions workflow provides two complementary jobs:

1. Header Validation (Cross-platform)

• Tests on Ubuntu, Windows, and macOS
• Uses CMake to validate header-only library compilation
• Ensures portability across different platforms

2. Windows Build Validation

• Validates Visual Studio project structure
• Tests all configuration combinations (Debug/Release × x86/x64)
• Handles platform mapping (x86 → Win32 for MSBuild)
• Provides clear documentation about WDK requirements for kernel samples

📋 Build Matrix Coverage

• Platforms: Linux, Windows, macOS (for headers)
• Configurations: Debug, Release
• Architectures: x86, x64
• Build Systems: CMake + MSBuild validation

🎯 Benefits

• ✅ Better cross-platform testing for the header-only library
• ✅ Faster feedback with parallel job execution
• ✅ Modern CI infrastructure with better GitHub integration
• ✅ Clear separation between header validation and Windows-specific builds
• ✅ Comprehensive project structure validation

The migration maintains all original AppVeyor functionality while adding cross-platform header testing capabilities, making the CI more robust for this header-only C++ testing framework.

Fixes #17.

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