Replaces the pause/resume message protocol on the worker-to-parent stream path with a shared Int32Array of two slots: INFLIGHT and STATE. Worker increments INFLIGHT after each emit and, on hitting HIGH_WATER, parks in Atomics.waitAsync(flags, INFLIGHT, current). Parent decrements INFLIGHT + Atomics.notify on each pull. Cancellation is STATE=CANCEL + notify; worker checks STATE after each wake. Eliminates: - 'pause'/'resume' postMessage round-trip - Adaptive setImmediate yield per-emit (no longer needed; worker parks directly on the atomic when backpressured, runs at microtask speed otherwise) Only wired through dispatchStream + handleStreamTask. pipeToPort and channel Distributor still use the message-based adaptive-yield path via the opts.flags-less branch in pipeIterable. Results (M-series mac, Node v24.15.0, noopStream, 100K items): before after stream throughput ~410K/s ~455K/s +11% backpressure buffer (avg) 184 ms 89 ms ~2x tighter backpressure buffer (max) 991 ms 96 ms ~10x tighter backpressure steady-state still stable growing at ~89ms Max queued items drops from ~200 to ~18 (HIGH_WATER + 1-2 overshoot), within a factor of 2 of the theoretical minimum. Follow-up work: apply the same pattern to pipeToPort (parent->worker iterable args) and channel Distributor (1:N fan-out). Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>