An image element may name the picture it shows (`assetId`); without
one, its own id names it, as before. A copy of an image can then be a
new element showing its original's picture, with nothing copied or
uploaded, and nothing to wait for while the original is still
uploading.

References, upload placeholders and cleanup all read the picture
through collectAssetIdFromElementPayload, so they follow. For builds
from before the field, page and strategy snapshots also list each
picture under the id of every image showing it, and a whole write that
leaves the field out keeps it: an image never changes picture.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>