nx_ir_eval_gate.nx

nx_ir_eval_gate.nx -- KAT for nx_ir_eval (the search-SOTA ruler). Native sovereign lane; exit 0 = all pass, N = assertion N failed. Every asserted value is either exact-by-construction (rank-1 discount is log2(2)=1.0; an already-ideal ranking has nDCG==1.0 because DCG==IDCG) or a plain integer ratio (MRR/Recall/Precision/AP) -- so the KAT verifies the summation/ordering logic bit-exactly without depending on fx_log2's internal bits for non-powers-of-two, which are checked by inequality/ordering instead.