NEXT STEP pushes the $250k precious refinery for base-metal gains the furnace already earns #419

refinery.js gainNext quotes every held bucket at level+1 and sums every positive gain, so with the smelting furnace built (next = precious refinery) copper, aluminum and steel gains count toward it. GROW comes before REFINE, so with 400 t of 97% copper and no gold or silver NEXT STEP says BUILD A PRECIOUS-METALS REFINERY $250,000 (gain $338,168, all copper); after building it the next step is REFINE COPPER, which the furnace could already do. Also seen in normal autoplay on an aluminum line. Fix: count only buckets the current level cannot refine (levelFor(m) === next).