Detect LCP images after soft navigations in NgOptimizedImage #71812

Which @angular/* package(s) are relevant/related to the feature request?

common

Description

NgOptimizedImage warns during development when an LCP image is not marked as priority. I think it would be useful to extend this diagnostic to soft navigations, so it also covers images displayed after navigating between routes without reloading the page.

e.g navigating from a product list to a product detail page can introduce a different LCP image.
Developers should receive the same missing-priority warning when the browser identifies that image as the LCP of the soft navigation.

Proposed solution

Extend LCPImageObserver to observe soft-navigation and interaction-contentful-paint entries when supported, associating them through interactionId.

Only paints associated with a detected soft navigation should trigger this diagnostic. An unrelated interaction that displays another large image should not be treated as a new navigation.

Keep the existing initial-page-load observation and use feature detection so browsers without these entry types continue to work as they do today.

Alternatives considered

I previously opened #69963 if someone can take care of it or suggest an better alternative

References

• Soft Navigations and Interaction Contentful Paint.
• Measuring soft navigations