feat: add thread safety to Bayes and LSI classifiers

[cardmagic]

Summary

• Add Mutex_m inclusion to both Bayes and LSI classifiers
• Wrap all public methods accessing shared state in synchronize blocks
• Add marshal_dump/marshal_load for serialization (Mutex can't be marshaled)

Problem

Concurrent train() and classify() calls corrupt internal state:

classifier = Classifier::Bayes.new('Spam', 'Ham')

threads = 10.times.map do
  Thread.new do
    100.times do
      classifier.train_spam("buy now cheap")
      classifier.classify("special offer")
    end
  end
end

threads.each(&:join)  # Corrupted state

Solution

Use the Mutex_m mixin (already a gem dependency) with a pattern that avoids deadlock:

def public_method(...)
  synchronize { private_unlocked_method(...) }
end

private

def private_unlocked_method(...)
  # actual implementation
end

Test plan

• All existing tests pass with bundle exec rake test
• All tests pass with NATIVE_VECTOR=true bundle exec rake test
• Marshal serialization still works (test_serialize_safe)

Closes #66