2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/166380Paper withdrawn, due a crucial error in the proof of Lemma 4.3: thus Theorems 1.2 and 1.3 remain unproven.15 pages, LatexCombinatoricsOptimization and Control05C78 (Primary) 68R10, 90C27 (Secondary)text