…#182)

* feat(logging): add unified logging functions with source_location support

This commit introduces inline logging functions that integrate with
GlobalLoggerRegistry and automatically capture source location information
at the call site.

Key features implemented:
- Level-specific functions: log_trace(), log_debug(), log_info(),
  log_warning(), log_error(), log_critical()
- Automatic source_location capture (C++17/C++20 compatible)
- Support for default logger and named loggers
- Utility functions: is_enabled(), flush()
- All functions are inline for zero overhead when logging is disabled

The log_functions.h header provides:
- log(level, message): Log to default logger
- log(level, message, logger): Log to specific logger instance
- log(level, message, logger_name): Log to named logger from registry
- Level-specific convenience functions with source location capture

Thread Safety:
- All functions are thread-safe
- Source location capture happens at compile-time
- Logger retrieval uses thread-safe GlobalLoggerRegistry

Updated common.h to include new logging headers for easy access.

Refs: #175

* test(logging): add comprehensive tests for unified logging functions and macros

This commit adds unit tests covering all aspects of the unified logging
implementation, ensuring correctness and thread safety.

Test categories implemented:

1. Basic Logging Function Tests:
   - Log message with correct level
   - Source location automatic capture
   - All log levels (trace, debug, info, warning, error, critical)

2. Named Logger Tests:
   - Logging to named loggers via registry
   - Logging to specific logger instances
   - Named logger with level-specific functions

3. Level Filtering Tests:
   - Log level filtering behavior
   - is_enabled() function correctness
   - Named logger level checking

4. Flush Tests:
   - Default logger flush
   - Named logger flush

5. Macro Tests:
   - LOG_* macros for all levels
   - LOG_*_TO macros for named loggers
   - LOG_IF conditional logging
   - LOG_FLUSH and LOG_IS_ENABLED utilities

6. Legacy Compatibility Tests:
   - THREAD_LOG_* macros redirect to LOG_*
   - All legacy levels work correctly

7. NullLogger Fallback Tests:
   - Unregistered logger returns NullLogger
   - No default logger uses NullLogger

8. Thread Safety Tests:
   - Concurrent logging to default logger
   - Concurrent logging to multiple loggers

9. String Compatibility Tests:
   - std::string_view input
   - const char* input
   - std::string input

All 30 tests pass successfully.

Refs: #175

* docs(logging): update API reference and changelog for unified logging

This commit updates documentation to reflect the new unified logging
functions and macros introduced in Issue #175.

API Reference updates:
- Added "Unified Logging" section with complete API documentation
- Documented log_functions.h with all function signatures
- Documented log_macros.h with all macro definitions
- Added usage examples for both functions and macros
- Documented compile-time level filtering via KCENON_MIN_LOG_LEVEL
- Documented legacy THREAD_LOG_* compatibility macros
- Updated version to 2.1 and last updated date

Changelog updates:
- Added entry for unified logging functions and macros
- Listed all new features:
  - Level-specific functions with source_location
  - Standard LOG_* macros
  - Named logger LOG_*_TO macros
  - Conditional LOG_IF macros
  - Compile-time level filtering
  - Legacy THREAD_LOG_* compatibility

Refs: #175

* fix(logging): resolve function overload ambiguity with Clang compiler

This commit fixes a compilation error on Clang where calls like
`log_info("message", "logger_name")` were ambiguous between:
- `log_info(string_view message, source_location loc = current())`
- `log_info(string_view message, string logger_name, source_location loc = current())`

The issue occurred because C string literals can implicitly convert to both
source_location (via default parameter) and std::string, causing Clang to
report "call to 'log_info' is ambiguous".

Solution:
- Renamed named logger functions from `log_*()` to `log_*_to()`
- Changed parameter order to `log_*_to(logger_name, message)`
- This eliminates overload ambiguity by using distinct function names

API changes:
- Old: `log_info("msg", "logger")` - REMOVED (ambiguous)
- New: `log_info_to("logger", "msg")` - ADDED (unambiguous)

The LOG_*_TO macros continue to work with the same syntax:
`LOG_INFO_TO("logger", "msg")` - unchanged interface

Updated:
- log_functions.h: Renamed functions with "_to" suffix
- log_macros.h: Updated macro implementations
- Tests: Updated to use new function names
- API documentation: Reflected new function signatures

Refs: #175