Subset of Material's MatStepper carrying the _iconOverrides
map. <cngx-mat-stepper> patches this map inside
afterNextRender to forward consumer-projected
<ng-template matStepperIcon> declarations through the wrapper
into Material's per-header indicator bindings (the only
deterministic forwarding path — <ng-content> projection
inside <mat-stepper> does not reach
@ContentChildren(MatStepperIcon) due to content-init
lifecycle ordering).