Menu links / link-field items to entities on another site are hidden by entity access

Problem/Motivation

A menu link or link-field item pointing to an entity that belongs to site B is rendered while browsing site A (or the default context). Core runs an access check on the target route; EntityAccessHooks::entityAccess() correctly returns forbidden ("Entity belongs to a different site"), so core drops the link from the menu tree / removes the field link. Result: cross-site links silently disappear ("missing menu links").

Proposed resolution

Site-scoped links already know their target site via the site_id / site URL option (set by SitesLinkitWidgetTrait), and SitesUrlGenerator already produces a correct absolute URL to the target site's hostname. Render these links as external/unrouted absolute URLs so the menu/link access system skips the entity-route access check entirely. EntityAccessHooks stays strict.

Integration points to cover:

• Link-field formatter: when an item carries site_id for another site, build the link as an external absolute URL.
• Menu links to site-scoped entities: same treatment so the menu tree keeps them.
• Active-trail suppression for cross-site links already exists in SitesLinkGenerator.