feat: add save/load methods for classifier persistence

[cardmagic]

Summary

• Add as_json method that returns a Hash representation
• Add to_json method that returns a JSON string
• Add from_json class method that accepts either a JSON string or Hash
• Add save(path) and load(path) for file operations
• Use versioned JSON format for future compatibility

Usage

# Train and save to file
classifier = Classifier::Bayes.new('Spam', 'Ham')
classifier.train_spam('buy now cheap')
classifier.save('model.json')

# Load from file and continue training
loaded = Classifier::Bayes.load('model.json')
loaded.train_ham('meeting tomorrow')
loaded.classify('special offer')  # => "Spam"

# Get hash representation
hash = classifier.as_json
# => { version: 1, type: 'bayes', categories: {...}, ... }

# Get JSON string
json_string = classifier.to_json
# => '{"version":1,"type":"bayes",...}'

# Load from JSON string
loaded = Classifier::Bayes.from_json(json_string)

# Load from hash (useful when JSON is already parsed)
loaded = Classifier::Bayes.from_json(hash)

Design Decisions

• JSON over Marshal: Human-readable, portable, version-safe
• LSI rebuilds on load: Only source data serialized, not vectors. Makes JSON portable across GSL/non-GSL environments
• Versioned format: {"version": 1, "type": "bayes|lsi", ...} allows future format changes
• Accepts both String and Hash: from_json handles both for flexibility

Test plan

• Round-trip tests for both Bayes and LSI
• Tests for as_json returning Hash
• Tests for from_json with both String and Hash
• Verify classifications match after save/load
• Test continued training on loaded classifiers
• All tests pass with bundle exec rake test
• All tests pass with NATIVE_VECTOR=true bundle exec rake test

Closes #17