A feature for provideA11yPreferences. Each with*
helper returns one of these carrying a hidden _target discriminator
the aggregator dispatches on. This mirrors the Select-family config
cascade (provideCngxSelect), but the aggregator reduces the axis
features to one scalar per axis (last-wins on duplicates) and forwards
it to the single-arg provide<Axis>(initial) rather than bucketing
same-kind features.