Referenced entities are exported once per reference

Problem/Motivation

doExportToArray() caches each exported entity, but on a cache hit it returns the whole payload again, so an entity reached N times in the reference graph is serialised N times.

Anything shared across the graph multiplies: the node author, the stores a product belongs to, a vocabulary's terms. A handful of entities referenced from everywhere can account for most of the file. On content with a wide reference graph, the export grows large enough that the download exhausts the PHP memory limit and the entity cannot be exported at all through the UI.

Proposed resolution

On a cache hit, export a reference instead of the payload: drop custom_fields and it becomes the stub shape that the importer already resolves by uuid. EntityReference.php emits exactly this when isReferenceCached() is true. Every entity is still exported in full exactly once so no data is lost.

User interface changes

None.

API changes

None. The emitted shape is already produced elsewhere in the module.

Data model changes

None.