Add the image and media image shapes

Problem/Motivation

Images are the most-used shape and the one with the most existing behaviour to preserve: derivatives by image style, alt and title, dimensions, and the default image case. There are two paths to the same output — a plain image field and an entity reference to an image media entity — and both appear across the sites.

Proposed resolution

• Port the existing image structure into an image shape, taking the style from the property settings.
• A media_image shape that resolves the media entity, finds its source field and delegates to the same builder, so the output is identical whichever way the site stores images.
• Validate the style name before a derivative is built, and keep the fix for the default image that used to return an empty alt and image_id: 0.
• Cache metadata from the file and the media entity, not just from the host entity.

Remaining tasks

Everything.