Summary

Document the feature flags framework in common_system including compile-time feature detection, runtime feature flags, and cross-system feature dependency management.

Parent Issue

Part of: [EPIC] docs: Address documentation gaps across all ecosystem systems (#325)

Background (Why)

common_system provides a feature flags subsystem with 4 header files that enable conditional feature activation across the ecosystem. This is important for:

• Gradual feature rollouts
• Platform-specific code paths
• Optional dependency management
• Build configuration

Source files:

• include/kcenon/common/config/feature_flags.h — Runtime feature flag management
• include/kcenon/common/config/feature_flags_core.h — Core flag definitions and types
• include/kcenon/common/config/feature_detection.h — Compile-time feature detection
• include/kcenon/common/config/feature_system_deps.h — Cross-system feature dependencies

Scope (What)

1. Feature Flags System Overview

• Compile-time vs runtime feature flags
• How feature flags flow through the ecosystem
• Flag lifecycle (define → activate → check → deprecate)

2. Core Definitions (feature_flags_core.h)

• Flag types (boolean, percentage-based, variant)
• Flag naming conventions
• Default values and overrides

3. Feature Detection (feature_detection.h)

• Compile-time platform detection (OS, architecture, compiler)
• C++ feature detection (__has_include, _cpp* macros)
• Hardware capability detection (SIMD: AVX2, NEON)
• How detection results feed into feature flags

4. Runtime Feature Flags (feature_flags.h)

• Flag registration and lookup API
• Dynamic flag evaluation
• Flag change notifications
• Thread-safe flag access

5. System Dependencies (feature_system_deps.h)

• How features declare cross-system dependencies
• Dependency resolution at build time
• Optional vs required dependencies
• Feature compatibility matrix

6. Usage Examples

// Compile-time
#if KCENON_HAS_FEATURE(SIMD_AVX2)
    // AVX2-optimized path
#endif

// Runtime
if (feature_flags::is_enabled("async_logging")) {
    // Async path
}

Acceptance Criteria

• All 4 feature flag headers documented
• Compile-time vs runtime distinction clear
• Feature detection capabilities listed
• Cross-system dependency model explained
• Usage examples for both compile-time and runtime
• Integration with CMake build system explained