Retry interrupted waits in io_wait and process_wait

[tavianator]

Fixes: #467

Summary

• Retry io_wait after a stale selector wake-up while preserving the original timeout.
• Retry process_wait when the selector returns false, indicating that the native wait was interrupted before process completion.
• Preserve nil from process_wait, including legitimate Process::WNOHANG results.
• Require io-event ~> 1.21, which provides consistent interrupted-process-wait behavior across native backends.
• Add backend-independent regression coverage for Async's retry policy.

Scope

io-event owns the native interruption and cancellation contract; Async owns the higher-level retry policy. socketry/io-event#215 fixed modern io_uring cancellation and was released in io-event v1.21.0.

The separate threaded process-wait behavior is not worked around here. socketry/io-event#212 adds test-only coverage on Ruby 4.0 and newer. Ruby 3.3 and 3.4 remain skipped there pending the CRuby backports:

• [Backport 3.3] Handle spurious wakeups in Thread#join ruby/ruby#18502 for Ruby 3.3
• [Backport 3.4] Handle spurious wakeups in Thread#join ruby/ruby#18503 for Ruby 3.4

Testing

• BUNDLE_MIRROR__RUBYGEMS__ORG=https://rubygems.org BUNDLE_GEMFILE=gems.rb bundle exec bake test
• 541 tests passed with 1,193 assertions on Ruby 4.0.6 using io-event 1.21.0.
• CI passes across CRuby, selector backends, worker-pool coverage, lint, documentation, CodeQL, and external tests.
• The JRuby timeout cascade and TruffleRuby Fiber.set_scheduler failure reproduce identically on the previous commit (57ffab4) and are unrelated to this update.

Types of Changes

• Bug fix.

Contribution

• I added tests for my changes.
• I tested my changes locally.
• I agree to the Developer's Certificate of Origin 1.1.

[samuel-williams-shopify]

The final split is now explicit: Async retries only selector-level stale wake-ups (io_wait and an explicit false from native process_wait). It preserves nil, including legitimate Process::WNOHANG results. The threaded fallback is not worked around in Async or io-event; socketry/io-event#212 adds Ruby 4.0+ regression coverage while ruby/ruby#18502 and ruby/ruby#18503 track the Ruby 3.3/3.4 backports.