nx_chan_mpmc_test.nx

nx_chan_mpmc_test.nx -- prove Vyukov MPMC channel under real multi-producer + multi-consumer contention. Test design: - hw_workers producer threads, each sends ITERATIONS_PER_PRODUCER messages (just their thread index repeated -- value content irrelevant; we count receipts). - hw_workers consumer threads, each drains until total received atomic counter hits expected total. - Atomic done-counter signals when all producers have finished pushing, so consumers know when to stop spinning on empty. PASS criteria: - total_received == producers * ITERATIONS_PER_PRODUCER (no lost messages, no duplicates) - chan_len returns 0 at end (queue drained) If Vyukov MPMC is broken (CAS race exposes wrong cell, ABA-style re-use), we'll see lost or duplicated messages -> total mismatch.