Rollback io_uring completion setup failures

[samuel-williams-shopify]

Summary

• distinguish acquired completion records from operations with prepared SQEs
• abandon and recycle completion state when SQE acquisition raises
• apply the rollback path to read, write, wait, and process-wait setup
• set cancellation state only after a cancellation SQE is available

Testing limitation

A deterministic regression requires forcing io_get_sqe() to raise after a completion record has been acquired. The initial test implemented that with a selector field and private native methods, but that placed fault-injection behavior in the released implementation. Those hooks and the test have been removed.

A future test could use a test-only native build or a Linux subprocess with liburing symbol interposition, provided it can expose the leaked completion state without adding production introspection.

Testing

• bundle exec bake test (296 passed, 21 skipped locally)
• Linux native compilation completed successfully in a container
• GitHub CI is running

[samuel-williams-shopify]

Updated based on review feedback: the selector-level SQE failure field, private test methods, and regression test have been removed. The PR now contains only the lifecycle fix. The PR description documents why deterministic coverage would require either a test-only native build or external liburing interposition plus non-production observability.