//! Domain layer — pure entities, value objects, and business rules.
//!
//! 6 primitive (v4 BP 採用):
//! - [`graph`] Node + Edge entity (open vocabulary)
//! - [`crud`] create / read / update / delete commands
//! - [`compute`] traversal + execution + constraint eval
//! - [`constraint`] edge-as-constraint evaluation
//! - [`autoversion`] append-only version chain
//! - [`specification`] first-class composable query object (BP: Specification pattern)