Complete the image tier outside the component

[romain-pm]

Part of #734. Three holes outside the component, each found by a real call site on Jahia/jahia.com.

• buildImageUrl registers no cache dependency. getImageProps does. buildImageUrl is the only option for a CSS background-image, so a background image silently loses the flush an <img> gets.
• No background mode. Add one that returns a ready url(...) value, with the same comma handling srcSet needs — a DAM URL containing a comma breaks url() the way it breaks srcSet (srcset URL rewriting corrupts candidate URLs containing commas jahia#23).
• readImageMeta is not exported. Layout.tsx on jahia.com re-reads j:width / j:height by hand, with its own try/catch, for og:image:width and og:image:height. The library already has that function, guarded, and keeps it private.
• No absolute URL. jahia.com hardcodes https://www.jahia.com six times for JSON-LD and reaches past the library to getAbsoluteUrl for og:image. Shared with the link side (buildNodeUrl gaps a link component cannot work around #756); land it once on buildNodeUrl and expose it on both.