Handle io_uring submission backpressure

[samuel-williams-shopify]

Summary

• add a deterministic regression test for transient submission backpressure
• return control to the event loop when io_uring_submit() reports EAGAIN or EBUSY
• allow queued completions to be processed before retrying pending submissions

Testing

• bundle exec bake test (296 passed, 21 skipped locally)
• the first commit contains the regression test before the fix