Summary

Add unified feature flag headers and normalize compiler/stdlib feature detection in common_system.

5W1H Specification

• Who: common_system maintainers
• What: Implement unified headers + KCENON_* macro exports
• Where: include/kcenon/common/config/ and config helpers
• When: TBD
• Why: Single source of truth for feature detection and integration flags
• How: New headers + CMake export + legacy aliases

Parent Issue

#223

Tasks

• Add feature_flags.h, feature_detection.h, feature_flags_core.h, feature_system_deps.h
• Guard __has_include usage and keep a fallback path
• Detect jthread via __cpp_lib_jthread / __cpp_lib_stop_token
• Export KCENON_* compile definitions via features.cmake or helper
• Provide legacy aliases behind a migration flag

Acceptance Criteria

• Unified headers compile standalone in common_system
• KCENON_HAS_SOURCE_LOCATION and KCENON_HAS_JTHREAD are accurate across compilers
• Legacy aliases do not override explicit 0 values