Install default SIGINT handler

[samuel-williams-shopify]

Summary

• Add a small Async::SIGINT compatibility shim loaded by require "async".
• Install the shim only on affected CRuby versions and only when SIGINT is still using the default handler.
• Add subprocess coverage for version gating, preserving existing handlers, and deferring SIGINT with Thread.handle_interrupt.

Testing

• bundle exec sus test/async.rb --verbose
• bundle exec rubocop -A lib/async.rb lib/async/sigint.rb test/async.rb
• bundle exec bake decode:index:coverage lib
• bundle exec sus

[samuel-williams-shopify]

I think fixing this upstream is a better idea.