Enqueue all n base intervals in order on the one stream and synchronize
once before returning (step(0): no launch, no wait). step_count, the raw
exchange ledger and an open window are published only after that wait;
any failure leaves step_count at its entry value and invalidates both
ledgers. Async device errors are no longer attributed to an interval;
the docstring says to use step(1) loops to localize one. Launch order and
buffers are unchanged. Adds a mocked-driver host test of the contract.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
