Add entity snapshot and rollback APIs to Koota

Normalized profile

What this task asks for

Add snapshot, rollback, and diff APIs for entities and worlds, including registry-based trait and relation serialization. Throws `Error` on duplicate keys, duplicate traits, or duplicate relations. The `relations` property is omitted entirely when the entity has no relations. Throws `Error` for destroyed entities or unregistered traits/relations. Throws `Error` if a relation target entity does not exist in the world. Throws `Error` for destroyed entities or unknown registry keys. `world.snapshot(registry)`, `world.rollback(registry, checkpoint)`, `entity.snapshot(registry)`, and `entity.rollback(registry, snapshot)` convenience methods must be added.