- Add AddClass method to add new classes after classifier creation
- Add RWMutex to protect Classes and datas from concurrent access
- Add locks to all public methods for thread safety:
  - RLock for read operations (LogScores, ProbScores, etc.)
  - Lock for write operations (Learn, Observe, AddClass, etc.)
- Add ErrClassExists and ErrAlreadyConverted error types
- Add comprehensive tests for AddClass functionality
- Add concurrent access tests to verify thread safety

All methods are now safe for concurrent use.

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Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>