feat(logging): add PSR-3 logger support

[usernane]

Description

secureLog() uses error_log() directly. Should support PSR-3 logger injection for integration with Monolog, etc.

Implementation Details

• Add optional Psr\Log\LoggerInterface dependency (suggest in composer.json)
• Add setLogger(LoggerInterface $logger) to AbstractHandler
• Modify secureLog() to use injected logger if available, fallback to error_log()
• Add Handler::setDefaultLogger() for global logger

Acceptance Criteria

• PSR-3 logger can be injected
• Backward compatible (works without PSR-3)
• Documentation updated