Handle interrupted io_uring process waits

[samuel-williams-shopify]

Summary

• preserve the native selector contract of returning false when process_wait is resumed before process completion
• safely cancel and drain io_uring waitid operations before stack-backed siginfo goes out of scope
• return false when cancellation wins, or preserve the real process status when waitid wins the cancellation race
• add shared coverage for interrupted native process waits

KQueue, EPoll, and legacy pidfd-based io_uring waits already return false on a normal interruption. This makes modern io_uring waitid consistent without imposing retry policy on io-event; higher-level schedulers such as Async can decide whether to retry.

Testing

• BUNDLE_GEMFILE=gems.rb bundle exec bake build
• BUNDLE_GEMFILE=gems.rb bundle exec bake test (292 passed, 17 skipped; 911 assertions)