Rust code can directly construct or modify ordinary models. Call validate() to check the current object; dump()/dump_json() revalidate it before serialization. These checks preserve field values. Normalization takes place during parsing.
For read-only access, use User::parse_validated_json(...) or Validated::new(user) to obtain a Validated<User> without DerefMut. into_inner() returns the ordinary model. Custom JsonType implementations with interior mutability are responsible for their own mutation rules.
