Add comprehensive feature specification for T4 to Roslyn Source Generator migration

[Copilot]

This PR provides a complete feature specification and implementation roadmap for migrating the Arke.ARI library's code generation system from T4 templates to modern Roslyn Source Generators.

Background

The current code generation system relies on T4 templates in the CodeGeneratror/ARICodeGen project, which has several limitations:

• Requires Visual Studio and .NET Framework dependencies
• Limited cross-platform support
• Complex maintenance due to T4 syntax
• Slower build times due to design-time template processing
• Poor IDE integration for template development

What This PR Adds

📋 Feature Specification (FEATURE-SPEC-T4-TO-ROSLYN-MIGRATION.md)

A comprehensive 40+ page specification covering:

• Current State Analysis: Detailed examination of the existing T4 template system
• Migration Goals: Performance, maintainability, and cross-platform objectives
• Technical Approach: Complete Roslyn Source Generator architecture
• 7-Week Implementation Plan: Phased approach with clear milestones
• Risk Assessment: Identified risks and mitigation strategies
• Success Criteria: Measurable goals including 20%+ build time improvement

🔧 Technical Implementation Guide (TECHNICAL-IMPLEMENTATION-GUIDE.md)

Detailed technical documentation with:

• Line-by-line T4 template analysis
• Complete Roslyn Source Generator code examples
• Schema parsing and type conversion utilities
• Project structure and integration patterns
• Comprehensive testing strategies

✅ Migration Checklist (MIGRATION-CHECKLIST.md)

Actionable implementation roadmap including:

• Phase-by-phase task breakdown
• Quality gates and validation checkpoints
• Rollback procedures if migration fails
• Weekly timeline with specific deliverables

🏗️ Example Source Generator (example-source-generator/)

Working prototype demonstrating:

• Proper project configuration for source generation
• Basic generator implementation with JSON schema parsing
• Integration patterns with the main Arke.ARI project

Key Benefits of the Migration

• 🚀 Performance: 20%+ faster builds through compile-time generation
• 🌍 Cross-Platform: Full Windows, macOS, and Linux support
• 🛠️ Better DX: Modern IDE support with IntelliSense and debugging
• 🔧 Maintainability: Simplified codebase using modern C# features
• 📦 Dependencies: Eliminates Visual Studio and T4 dependencies

Generated Code Impact

The migration will maintain 100% API compatibility while modernizing the generation process. The current system generates:

• Models: 50+ data transfer objects from JSON schemas
• Actions: REST API client interfaces and implementations
• Events: WebSocket event classes with proper inheritance
• Client: Main ARIClient with event handling and dependency injection

Implementation Timeline

Phase	Duration	Key Deliverables
Foundation	2 weeks	Generator infrastructure, schema parsing
Model Generation	1 week	Model classes with inheritance support
Action Generation	1 week	REST API client generation
Event/Client	1 week	Event handling and main client
Integration	1 week	Full testing and validation
Migration	1 week	T4 removal and documentation

Next Steps

This specification provides the complete roadmap for implementation. The next phase would be:

1. Review and approve the technical approach
2. Allocate development resources for the 7-week project
3. Begin Phase 1 implementation with the foundation infrastructure

The migration will significantly modernize the Arke.ARI library while maintaining full backward compatibility and improving the developer experience for both library maintainers and consumers.

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[srs-adamr]

@copilot Can you also write unit tests for your PR changes?