Add RBS type signatures

[cardmagic]

Summary

Add RBS type signatures to enable static type checking and improved IDE support.

Motivation

• Ruby 3.0+ includes RBS as the standard type signature format
• Enables static analysis with tools like Steep
• Improves IDE autocompletion and documentation
• Helps catch type errors before runtime

Scope

Type signatures needed for:

• Classifier::Bayes - initialize, train, untrain, classify, classifications
• Classifier::LSI - initialize, add_item, classify, search, find_related
• Classifier::ContentNode - internal LSI data structure
• String extensions - word_hash, clean_word_hash
• Vector/Matrix extensions - normalize, magnitude, SV_decomp

Files to create

sig/
  classifier.rbs
  classifier/bayes.rbs
  classifier/lsi.rbs
  classifier/lsi/content_node.rbs
  classifier/extensions/string.rbs
  classifier/extensions/vector.rbs

References

• https://github.com/ruby/rbs
• https://github.com/soutaro/steep