…174) (#181)

* feat(interfaces): implement GlobalLoggerRegistry for runtime binding

This commit introduces the GlobalLoggerRegistry class to resolve the
circular dependency between thread_system and logger_system. The registry
provides a centralized, thread-safe mechanism for managing logger instances
across all subsystems.

Key features implemented:
- Thread-safe singleton pattern using Meyer's singleton idiom
- std::shared_mutex for efficient read/write locking
- Default and named logger management
- Factory-based lazy initialization for deferred logger creation
- NullLogger fallback for unregistered logger requests
- Convenience functions (get_registry, get_logger)

The NullLogger class provides a safe no-op implementation that:
- Returns success for all logging operations
- Always reports log level as 'off'
- Ensures code functions silently when logging is not configured

Public API:
- register_logger(name, logger): Register a named logger
- get_logger(name): Retrieve a named logger (returns NullLogger if not found)
- unregister_logger(name): Remove a named logger
- set_default_logger(logger): Set the default logger
- get_default_logger(): Get the default logger (returns NullLogger if not set)
- register_factory(name, factory): Register lazy initialization factory
- set_default_factory(factory): Set factory for default logger
- has_logger(name): Check if logger/factory exists
- has_default_logger(): Check if default logger/factory exists
- clear(): Remove all registered loggers and factories
- size(): Get count of registered named loggers

This implementation is part of Issue #174 and blocks Phase 2/3 work on
the runtime binding migration.

Refs: #174

* test(interfaces): add comprehensive tests for GlobalLoggerRegistry

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

Test categories implemented:

1. Singleton Tests:
   - Instance returns same reference
   - get_registry() convenience function works correctly

2. Default Logger Tests:
   - Returns NullLogger when no default is set
   - set_default_logger() succeeds with valid logger
   - set_default_logger() fails with null
   - Convenience function get_logger() returns default

3. Named Logger Tests:
   - register_logger() with valid parameters
   - Empty name validation
   - Null logger validation
   - Logger replacement behavior
   - get_logger() retrieval
   - NullLogger fallback for unregistered names
   - unregister_logger() behavior

4. Factory Tests:
   - Lazy initialization pattern
   - Factory called only once
   - Factory validation (empty name, null factory)
   - Factory vs existing logger precedence
   - register_logger() removes associated factory
   - Default factory lazy initialization

5. NullLogger Tests:
   - All operations succeed
   - Same instance returned
   - Always reports disabled

6. Clear and Size Tests:
   - clear() removes all state
   - size() returns correct count

7. Thread Safety Tests (35 test cases total):
   - Concurrent registration (10 threads, 100 loggers each)
   - Concurrent retrieval (10 threads, 1000 retrievals each)
   - Concurrent factory creation (race condition test)
   - Concurrent default logger access
   - Mixed read/write operations

8. Integration Tests:
   - Logging through registry
   - Multiple named loggers

All tests pass with no race conditions or deadlocks detected.

Refs: #174

* refactor(common): add logger interfaces to aggregated header

This commit adds the logger-related headers to the main common.h
aggregated header file for convenient access.

Added includes:
- interfaces/logger_interface.h: Core ILogger interface and log types
- interfaces/global_logger_registry.h: GlobalLoggerRegistry singleton

This allows users to access the logging infrastructure with a single
include of <kcenon/common/common.h> instead of having to include
individual header files.

Refs: #174

* docs(interfaces): add GlobalLoggerRegistry API documentation

This commit updates the documentation to include the new GlobalLoggerRegistry
class and related components.

Changes to API_REFERENCE.md:
- Updated ILogger section with correct header path and complete interface
- Added comprehensive GlobalLoggerRegistry documentation:
  - Singleton access pattern
  - Default logger management methods
  - Named logger management methods
  - Factory support for lazy initialization
  - Utility methods (clear, size, null_logger)
  - Convenience functions
  - Usage examples with code samples
- Added NullLogger documentation

Changes to CHANGELOG.md:
- Added GlobalLoggerRegistry to [Unreleased] section
- Listed key features: thread-safe singleton, named loggers, factory support
- Referenced Issue #174

Refs: #174