refactor: Enhance thread safety in logging module

[privateip]

• Add threading.RLock for thread-safe sensitive data filtering
• Protect filtering check with reentrant lock in log() function
• Create snapshot of logger names in _get_loggers() to prevent race conditions
• Fix fatal() to write to stderr instead of stdout
• Accept "NONE" string in set_level() for disabling logging
• Move initial logger level setting to after NONE constant definition
• Update docstrings to document thread-safety guarantees
• Update tests to verify stderr output in fatal function