0.17.0 — multiclass cross features

Cross features now cover multiclass classification: the same validation-selected difference/product columns regression and binary get (top numeric pairs of the base fit, raced audition at selection_rounds, judged on softmax validation log loss). Auto-on under the same gates (>= 2000 rows, >= 2 numeric features); explicit cross_features=True no longer raises on multiclass. Regression and binary paths are bit-identical to 0.16.1.

Full experiment record: benchmarks/M1_PLAN.md.