Subtract per-element mean before FWHT to improve quantization
No benefit for turbo3 (values already near-zero-mean after L2 normalization + FWHT). CATASTROPHIC for turbo4 — QJL residual is computed relative to the DC-removed signal but decoded without DC restoration, breaking the correction entirely. WARNING: any pre-processing that changes the signal before quantization must be perfectly inverted during dequant, including through the QJL path. HadaNorm mean-centering (originally planned as separate experiment) has the same incompatibility.