Negotiate optional io_uring setup flags

[samuel-williams-shopify]

Summary

• add a deterministic regression test for runtime setup-flag compatibility
• retry queue initialization while removing unsupported optional flag groups
• preserve the dependency between DEFER_TASKRUN and TASKRUN_FLAG
• use the same negotiation for selector initialization and support detection

Testing

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