Brotli::Enc::ZopfliNode

backward_references_hq.h:20. C packs {length, distance, dcode_insert_length, u.cost/shortcut/next} into 16 bytes with a union for the last field; the union aliases cost with shortcut/next, but every read happens before the aliasing write (EvaluateNode saves the cost first, ComputeShortestPathFromNodes runs after all cost reads), so separate fields preserve the semantics exactly.