nx_pipeline.nx

nx_pipeline.nx -- N-stage MIMD pipeline with bounded backpressure. Each stage runs the same number of workers; each worker pulls from the stage's input MPMC channel, applies the stage's transform fn, pushes to the stage's output channel. Bounded channels between stages naturally provide backpressure: when the downstream queue fills, upstream's nx_chan_send blocks (spin-on-full today; futex-blocking when nx_chan grows that). Shutdown: caller calls nx_pipeline_finish which pushes n_workers_per_stage sentinels (NX_PIPE_SENTINEL = i64::MIN) into stage-0's input channel; each worker that pulls a sentinel forwards one sentinel to the next stage and exits. Stage K receives exactly n_workers sentinels, so each of its workers gets one and exits. Requires uniform worker count across stages -- elastic per-stage sizing is a follow-up evolution. Composes against: [[nx_thread_pool_shared_queue]] (worker execution), [[vyukov_mpmc_channel]] (per-stage queue), [[nx_hw_dynamic_probes]] (worker sizing default), [[fn_ptr_indirect_call]] (typed transform dispatch).