UserLike

The common surface of a user-valued reference. Resolves to a concrete User when the referenced user still exists, or to a MissingUser tombstone when the id is orphaned — diver tables store user ids with no foreign key into app.users, so deleting a user leaves dangling references. Modelled as an interface (rather than a union) so callers can select the shared display fields without per-variant inline fragments, and so it can absorb more fields non-breakingly as more User-valued fields adopt it.