Application-root configuration cascade for the accordion. Pass any
combination of withAccordionLabels / withDefaultHeadingLevel features in
bootstrapApplication's providers array.
Resolution priority (high -> low):
Per-instance Input binding.
provideAccordionConfigAt(...) in a parent component's viewProviders.
provideAccordionConfig(...) at the application root.
Library defaults (CNGX_ACCORDION_DEFAULTS).
The provider merges supplied features with the library defaults so consumers
only declare keys they want to override.