- Add #[data] attribute for struct fields that aren't node parameters
  - Data fields are initialized with Default::default()
  - Passed as references to the underlying function
  - Excluded from registry metadata (internal state)
  - Generic types in data fields allowed without #[implementations]

- Add #[serialize] attribute for custom Node::serialize() implementation
  - Receives references to all data fields
  - Generates serialize() method in Node trait impl

- Conditional derives based on data field presence
  - With data fields: Debug, Clone only
  - Without data fields: Debug, Copy, Clone, PartialEq, Eq, PartialOrd, Ord, Hash