Component templates and instance overrides

A template defines a reusable component, and users create instances that override some fields, such as text or color. Design a compact JSON-like representation and the classes around it so that an instance stores only its overrides, not copies of the defaults. When the template author publishes a new revision, an instance owner can preview and apply it: compatible overrides survive, while overrides of fields that were removed or changed type are reported as conflicts and nothing changes until they are resolved.