Add timeout support to Condition#wait

[samuel-williams-shopify]

Summary

• add a timeout: keyword to Async::Condition#wait, inherited by Async::Notification
• raise Async::TimeoutError when the timeout expires
• preserve successful signal(nil) by wrapping delivered values internally
• add shared coverage for both condition implementations and an Unreleased note

This allows callers to write notification.wait(timeout: 1) directly instead of wrapping the wait with Task#with_timeout.

Validation

• bundle exec sus — 546 tests, 1,202 assertions
• RuboCop passes for all changed Ruby files
• git diff --check

The repository-wide RuboCop run still reports five pre-existing offenses under external/. The Decode documentation coverage task also currently crashes inside Decode while parsing the repository.