Modernize incremental generator: Remove LINQ, optimize performance, add acknowledgments

[Copilot]

Modernizes the incremental source generator with significant performance optimizations and project improvements while maintaining the exact same API and functionality.

Performance Optimizations

• Eliminated LINQ usage in hot code paths, replacing with efficient loops for better performance
• Implemented StringBuilder for memory-efficient code generation instead of string concatenation
• Removed 9 legacy files (708 lines of code) that supported the old visitor pattern but were no longer used
• Maintained raw string literals with interpolation for clean, readable generated code

Project Improvements

• Added copyright acknowledgments for PolySharp, Microsoft.CodeAnalysis, and Splat dependencies
• Added AI tools disclosure noting use of GitHub Copilot with human review and testing
• Enhanced README documentation with comprehensive details about the modern 4-stage incremental pipeline architecture

Architecture Benefits

The generator now fully leverages:

• True incremental compilation with cache-friendly immutable records
• Memory-efficient processing with early filtering and minimal allocations
• Independent stage caching for maximum performance benefits
• Immediate IDE feedback during editing in Visual Studio 17.10+

All existing Register<>(), RegisterLazySingleton<>(), and RegisterConstant<>() methods continue to work exactly as before with improved performance.

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