timeseries is uninstallable on s390x/ppc64le due to mandatory CatBoost dependency

autogluon.timeseries force-installs autogluon.tabular[catboost,...] (timeseries/setup.py:49). CatBoost only ships wheels for x86_64 and aarch64, so pip install autogluon.timeseries fails on s390x/ppc64le — even though Tabular treats CatBoost as optional.

Note: this can't be fixed by simply moving CatBoost to extras_require. CatBoost is the default estimator for the covariate regressor and per-step tabular models (model_name="CAT"), e.g.:

• configs/hyperparameter_presets.py:44
• regressor.py:77
• models/autogluon_tabular/per_step.py:125
• models/ensemble/array_based/models.py:41

Dropping it from the dependency closure would trade an install-time error for a runtime error under default presets on those platforms.

Fix requires both:

1. A platform marker on the catboost pin (; platform_machine in "x86_64 aarch64"), and
2. A fallback default (e.g. "GBM"/"XGB") when CatBoost is unavailable — contingent on LightGBM/XGBoost having wheels for those arches.