nx_rrf_test.nx

nx_rrf_test.nx -- KAT for Reciprocal Rank Fusion. Native sovereign lane; exit 0 = pass, N = assertion N failed. 3 items, 2 rankers: A=[0,1], B=[1,2]. item1 ranks in BOTH (rank2 in A + rank1 in B) -> highest fused; item0 only in A (rank1); item2 only in B (rank2). So fused: item1 > item0 > item2, and argmax == item1.