nx_chan.nx

nx_chan.nx -- Vyukov MPMC bounded lock-free queue (i64 messages). Multi-Producer Multi-Consumer queue based on Dmitry Vyukov's 2010 design ("Bounded MPMC queue", 1024cores.net). Each cell carries a sequence number; producer sees `seq == enq_pos` to claim a slot, consumer sees `seq == deq_pos + 1` to drain. CAS bumps the position counter. Wait-free under no contention, lock-free under contention. Properties: * MPMC: any number of producers + any number of consumers * Bounded: capacity rounded up to power of 2 (mask-wrap) * Lock-free: no spin locks, no futex waits in fast path * FIFO order globally Compose-against: [[atomic_intrinsics_real_amo]] for CAS+FAA; [[thread_clone_native_trampoline]] for the worker threads; [[nx_hw_dynamic_probes]] for sizing producer/consumer count. Predecessor nx_channel.nx remains for SPSC code (smaller cell footprint, no per-cell seq); use nx_chan.nx whenever multiple threads can produce or consume. Reference: Vyukov D., "Bounded MPMC queue", 2010 (independent rederive from algorithm description; no code copied).