Add graceful default signal handlers

[samuel-williams-shopify]

Summary

Adds async/signals/graceful, which immediately installs default SIGINT and SIGTERM handlers that raise Interrupt on Thread.main. Existing non-default traps are preserved.

This provides a shared graceful shutdown policy for libraries like Async and Async::Container without each caller needing to install its own process signal traps.

Validation

• bundle exec rubocop -A
• bundle exec sus test/async/signals/graceful.rb --verbose
• bundle exec bake decode:index:coverage lib
• COVERAGE=PartialSummary bundle exec sus
• bundle exec bake covered:validate --paths .covered.db

[samuel-williams-shopify]

I dislike this approach.

[samuel-williams-shopify]

This is the lesser of two slightly problematic approaches.