nx_dr_chain_gate.nx

nx_dr_chain_gate.nx -- KAT + neg-controls for the end-to-end deep-research chain (DR-CHAIN). Scenario: 3 groundtruth insights, 3 source docs with different coverage and cost. doc0 "alpha one beta two" covers insight0+1 cost 2 doc1 "gamma three extra words here" covers insight2 cost 3 doc2 "alpha one" covers insight0 cost 1 Reading everything would recall 3/3 = 1000. Under budget 3 the conductor picks doc0+doc2 (best value) and drops doc1 -- the ONLY source for insight2 -- so recall_selected = 666. That delta IS the measured cost of the opportunity-cost constraint. Neg-controls prove the drop is budget-caused (unlimited budget restores 1000) and that selecting nothing scores 0.