Contao native frontend cache or Symfony HTTP cache for shared-content invalidation

Goal: Keep anonymous visitor HTML current when a shared content element, such as a navigation item or news teaser, is edited in the Contao backend and displayed on multiple pages.

Constraint: Contao's native frontend cache clears the originating page on save but may not invalidate every dependent page. An external Symfony HTTP cache or Varnish proxy requires separate purge keys and does not automatically receive Contao's backend cache-clearing signals. Behavior can differ between Contao 4.x LTS and 5.x depending on installed extensions.

• Which approach provides more reliable invalidation for content reused across pages?
• Does Contao's native cache clear all dependent pages automatically, or only the edited page?
• What purge integration is required when an external reverse proxy is preferred?