App-wide configuration cascade for the table-of-contents organism. Resolves
in priority order:
Per-instance Input binding.
provideTocConfigAt(...) in a parent component's viewProviders.
provideTocConfig(...) at the application root.
Library defaults (this token's factory).
providedIn: 'root' with a default factory means consumers never need to
provide the token explicitly - inject(CNGX_TOC_CONFIG) always resolves.
Consumers who want overrides call provideTocConfig(...) in
bootstrapApplication providers (root cascade) or provideTocConfigAt(...)
in a component's viewProviders (sub-tree cascade).