Alternative to: #72797

Fit text did not work after navigating to a page using the Interactivity API router. When users navigated to a post via client-side navigation (e.g., in a Query block with pagination), fit text blocks would not recalculate their font sizes, appearing either too large or too small.

This occurred because the fit text initialization only ran on the window.load event, which doesn't fire during client-side navigation.

This PR refactors fit text front end to be an interactivity api first implementation, using an interactivity api init event instead of a window load event.

cc: @luisherranz

Changes

1. Updated fit-text-frontend.js:

   • Added Interactivity API store with init callback.
   • Callback uses data-wp-init directive to initialize fit text on element mount.
   • Returns cleanup function to disconnect ResizeObserver and remove style elements when elements are removed.

2. Updated typography.php:

   • Automatically injects data-wp-interactive="core/fit-text" directive on blocks with fit text enabled.
   • Adds data-wp-init="callbacks.init" to trigger initialization.
   • Uses WP_HTML_Tag_Processor to safely modify block HTML.

3. Updated package.json:

   • Added @wordpress/interactivity dependency to @wordpress/block-editor

Testing Instructions

1. Enable the "Interactivity API: Full-page client-side navigation" experiment.
2. Add a new post with fit-text, load the blog page, and see that the post with fit-text works well.
3. Navigate to that post (e.g., click the post title) and verify fit-text still looks as expected (on trunk, it does not work).