Add LSI benchmark system for GSL vs native Ruby comparison

[cardmagic]

Summary

Add a benchmark system to measure and compare LSI performance between GSL and native Ruby implementations.

Features

• benchmark/lsi_benchmark.rb script for performance testing
• rake benchmark task to run with current configuration
• rake benchmark:compare task to compare GSL vs native Ruby

Benchmark Operations

• add_items - Adding documents to the index
• build_index - Building the SVD index
• classify - Classification performance
• search - Search query performance
• find_related - Finding related documents

Related

PR #74