nx_dr_fuse_gate.nx

nx_dr_fuse_gate.nx -- KAT + neg-controls for the learned signal combiner (DR-12). Synthetic set where the right answer is KNOWN: the "semantic" feature separates insights from distractors, the "lexical" feature is deliberately ANTI-correlated (a lexical-only weighting scores 0), and the third feature is constant noise. A fitter that works must discover the informative feature and ignore the misleading one. Proves in particular: (1) weights fitted on a TRAINING task range generalise to a DISJOINT held-out task, and (2) the NEG-CONTROL -- on a task whose insights and distractors are IDENTICAL, recall at matched precision is 0, so the metric cannot be gamed by any weighting.