Document child signal handling

[samuel-williams-shopify]

Summary

• Document why forked children are created from a short-lived Thread.new.
• Document why child execution resets SignalException handling after Thread.new.
• Add regression coverage for the existing idempotent Channel#close behavior.

Testing

• bundle exec sus --verbose test/async/container/channel.rb
• bundle exec rubocop lib/async/container/channel.rb test/async/container/channel.rb
• bundle exec sus --verbose