Wires the 3-stage template cascade for the <cngx-stepper> skin
slots: per-instance directive > CNGX_STEPPER_CONFIG.templates.<key>
null (built-in default).
Pure - no DI, no side effects. Safe to call from a field-init
block. Sibling of createTabOverflowTemplateBindings and the
family-wide createTemplateRegistry.
Tabs has no parallel slot surface yet - see
stepper-accepted-debt ยง3 for the planned Phase-4 closure.