Unified aggregator for the stepper family. Filters by _target and
forwards: config features to provideStepperConfig, i18n
features to provideStepperI18n. Mirrors provideCngxTabs /
provideCngxSelect.
Returns EnvironmentProviders for app-root use; the
component-scoped twin provideCngxStepperAt returns
Provider[] because viewProviders cannot accept opaque
EnvironmentProviders.