What

This PR improves the Navigation Link block in two key ways:

• makes Navigation Link blocks' URL attribute dynamic if the link is to an entity (e.g. Page, Post...etc) using the Block Bindings API
• makes it clear when a Nav Link block is linking to an entity vs a static URL through improved UI indicators

Closes #18345

Why

The Core Problem

The Navigation Link block has never dynamically fetched URLs for linked entities (e.g. Pages, Posts):

• When a user selects a Page/Post in the Editor, the block stores:
  • id: the ID of the entity.
  • url: a static copy of the permalink at that time.
• This url remains fixed — even if the entity's actual permalink changes later.
• Critically, the front end also uses this static URL. There is no dynamic lookup on render, which was previously assumed.

This means:

• If you change a page's URL (e.g. change the slug), existing Nav Links break silently.
• There is a false sense of safety — users believe they are linking to an entity, but the system never tracks or updates the link.

Why This Is a Major Usability Issue

1. WYSIWYG Mismatch

   • Editor shows outdated links.
   • Front end shows exactly the same outdated links.
   • This contradicts user expectations and introduces subtle bugs.

2. Invisible Link Types

   • There's no visual difference between a link to a Page and a manually entered static URL.
   • Users can't tell if their link is "safe" (entity-bound) or "fragile" (static).

3. Poor Content Resilience

   • If you rename or move content, your menus don't adapt.
   • This leads to 404s and broken navigation — especially problematic on large or client-managed sites.

How

Using the Block Bindings API, we bind the url attribute of the block to the entity represented by the id attribute of the block through a new core/entity binding source.

Key implementation details:

• Created a generic core/entity binding source that uses source_args['key'] to determine which property to return (e.g., url)
• Updated the Navigation Link block to automatically create bindings when an entity is selected
• Enhanced the LinkControl UI to clearly indicate when a link is bound to an entity
• Disabled manual URL editing for bound links with an "unlink" option to break the binding
• Updated PHP rendering to use the Block Bindings API for automatic URL resolution

This creates a unified approach between editor and frontend, with clear visual indicators for dynamic vs static links.

Note: After discussion with subject matter experts (@ockham and @cbravobernal), we agreed to implement a generic core/entity binding source for now. @ockham will investigate the possibility of normalizing this with existing data sources in the future.

Proposed Followups

The following items are being pushed to immediate follow ups:

• update design to follow mockups in this comment
• automatically update the Text content of the Nav Link item to match the chosen entity. This is the most likely workflow.
• update the Submenu block to match the implementation. Requires a quick refactor to align and share code between the blocks first to make this refactor easier.
• remove entity block binding and utilise existing post-data and term-data bindings once they have been made more extensible (see this comment)
• Avoid using state to manage focus