Add scoped signal events.

[samuel-williams-shopify]

Summary

• Use async-signals for scoped controller signal trap installation instead of the local Async::Container::Signals helper.
• Queue trapped controller signals as preallocated Controller::SignalEvent objects so trap handlers only wake the controller event loop.
• Add Async::Container::Events, a pipe-backed Thread::Queue wrapper that can wake IO.select.
• Thread controller events through container/group waits and startup/restart readiness waits so signal events can interrupt blocking child waits deterministically.
• Avoid exceptions on event pipe wake/drain by using IO#write_nonblock(..., exception: false) and IO#read_nonblock(..., exception: false).
• Remove the temporary child GC.start workaround now that io-event v1.17.0 fixes inherited selector ownership after fork.

Testing

• /Users/samuel/.local/state/tec/toolchain/base_profile/bin/bundle exec sus test/async/container/controller.rb
• /Users/samuel/.local/state/tec/toolchain/base_profile/bin/bundle exec sus
• /Users/samuel/.local/state/tec/toolchain/base_profile/bin/bundle exec bake test:external