Harden io_uring submit and park against CQ-overflow backpressure#81
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Under IORING_FEAT_NODROP the kernel returns EBUSY when the CQ is full of unreaped completions - the same overflow that terminates the doorbell multishot. submitIoSlow asserted submit >= 0 and aborted; defer instead, leaving the SQEs queued and lastSubmitCycles stale so the service loop drains the CQ, re-submits, and the staleness gate retries. Allow EBUSY in parkThread's enter2 assert too. Also: count IO_ENQUEUED by what submit consumed, not pre-submit sq_ready; and assert one-shot completions so a future multishot IO op trips instead of double-completing.
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Under IORING_FEAT_NODROP the kernel returns EBUSY when the CQ is full of unreaped completions - the same overflow that terminates the doorbell multishot. submitIoSlow asserted submit >= 0 and aborted; defer instead, leaving the SQEs queued and lastSubmitCycles stale so the service loop drains the CQ, re-submits, and the staleness gate retries. Allow EBUSY in parkThread's enter2 assert too.
Also: count IO_ENQUEUED by what submit consumed, not pre-submit sq_ready; and assert one-shot completions so a future multishot IO op trips instead of double-completing.