Support modern Ruby syntax

[zeisler]

Summary

• modernize the gem for Ruby 3.1 through 4.0 and add CI coverage
• parse current Ruby grammar through Prism with safe atomic fallbacks
• add structured graphs for loops, exceptions, pattern matching, blocks, and terminal flow

Verification

• bundle exec rake spec (103 examples, 0 failures; 2 existing pending)
• bundle exec rake build