nx_sig2_bp_gate.nx

nx_sig2_bp_gate.nx -- ADVERSARIAL gate for nx_sig2_txq (the relay backpressure fix). Reproduces the EXACT field failure shape the old path had: a receiver that stops reading while the sender floods video-sized frames. OLD behavior: blocking write -> the whole relay stalls (head-of-line), or a partial write tears a frame and poisons the stream. REQUIRED behavior, proven here on REAL sockets with a tiny SO_SNDBUF: T1 NO-STALL : 300 x 8KB sends to a wedged receiver return in bounded wall time; some accepted, the rest dropped WHOLE (never blocks, never tears). T2 INTEGRITY : when the receiver resumes, EVERY accepted frame arrives byte-perfect, in order, exactly once -- the stream parses cleanly end-to-end (no torn frames possible). T3 FAIL-STOP : sends after the receiver dies report fatal (conn reap), no crash (SIGPIPE ignored). T4 FAST PATH : with a live reader, frames go direct (queue stays empty) and all arrive intact. license_tier: ORIGINAL