Cache RBS core index in indexer test setup

[st0012]

Summary

• Cache the RBSIndexer#index_ruby_core result using Marshal.dump/Marshal.load in RubyIndexer::TestCase#setup, so it's built once per test class instead of once per test method
• Previously, every indexer test method (~264 across 9 test classes) rebuilt the entire Ruby core type index from scratch, adding ~2 minutes of pure setup overhead

Benchmarks

Local (bundle exec rake test:indexer, Apple Silicon, Ruby 3.4.7, 3 runs each):

	Run 1	Run 2	Run 3	Avg
Before	166.1s	124.3s	111.5s	133.9s
After	21.8s	21.5s	23.2s	22.1s