Closes #327

Summary

Add comprehensive documentation for the feature flags framework (docs/FEATURE_FLAGS_GUIDE.md) covering all 4 header files:

• feature_flags_core.h — Preprocessor helpers, compiler detection, platform detection, C++ standard detection
• feature_detection.h — C++20/23 standard library feature detection (15+ features)
• feature_flags.h — Main entry point with legacy aliases and static assertions
• feature_system_deps.h — Cross-system integration flags for 7 ecosystem systems

Documentation Contents

1. Feature Flags System Overview

• Compile-time vs runtime feature flags distinction
• Flag lifecycle (define → activate → check → deprecate → remove)
• Architecture diagram showing integration with ecosystem

2. Core Definitions (feature_flags_core.h)

• Preprocessor helpers: KCENON_HAS_INCLUDE, KCENON_HAS_CPP_ATTRIBUTE, KCENON_HAS_FEATURE, KCENON_HAS_BUILTIN
• Compiler detection: MSVC, Clang, GCC with version numbers
• C++ standard detection: C++17, C++20, C++23
• Platform detection: Windows, Linux, macOS, Unix
• Legacy aliases: Backward compatibility support

3. Feature Detection (feature_detection.h)

• C++20 features: source_location, jthread/stop_token, format, span, ranges, concepts, coroutines, three-way comparison
• C++23 features: expected, stacktrace, optional monadic operations
• Constexpr enhancements: constexpr vector, constexpr string
• Platform-specific features: SIMD detection (AVX2, NEON)

4. Runtime Feature Flags (feature_flags.h)

• Flag registration during initialization
• Dynamic evaluation throughout application
• Flag change notifications via hot-reload
• Thread-safe access patterns

5. System Dependencies (feature_system_deps.h)

• 7 system integration flags: thread_system, logger_system, monitoring_system, container_system, network_system, database_system, messaging_system
• Dependency resolution at build time
• Optional vs required dependencies
• Feature compatibility matrix

6. Usage Examples

• Compile-time examples: Platform-specific code, C++ feature detection, compiler optimizations
• Runtime examples: Feature toggles, gradual rollouts
• Cross-system integration: Logger + Thread, Database + Monitoring

7. CMake Integration

• Build configuration with feature flags
• System integration options

8. Migration Guide

• Migrating from legacy macros to KCENON_* macros
• Gradual migration strategy for large codebases
• Deprecation timeline

9. Troubleshooting

• Feature not detected
• System integration not working
• Legacy macro conflicts
• C++17 static assertion failure

Test Plan

• Documentation follows existing style (CONFIG_GUIDE.md, INTEGRATION_GUIDE.md)
• All 4 feature flag headers documented
• 15+ C++20/23 features covered
• 7 system integration flags documented
• Compile-time and runtime examples provided
• CMake integration guide included
• Migration guide from legacy macros
• Troubleshooting section added

Related Files

• include/kcenon/common/config/feature_flags_core.h
• include/kcenon/common/config/feature_detection.h
• include/kcenon/common/config/feature_flags.h
• include/kcenon/common/config/feature_system_deps.h