Focus-driven group-collapse policy for grouped <cngx-stepper> flows.
'expand-active' collapses every non-active cngxStepGroup branch to
its header node and expands only the group holding the active step;
'off' (default) keeps every group's children rendered in the strip -
the browser-native baseline. Orthogonal to the mobileCollapse axis;
the two never edit each other. The Material twin ([cngxMatStepper])
ignores this setting.