domain: add Identity & Access entities (User, Role, Permission, Group)

crates/domain/tests/entities/user.rs

@@ -0,0 +1,11 @@
+use domain::entities::User;
+use domain::value_objects::{Email, PasswordHash};
+
+#[test]
+fn creates_user_with_unique_id() {
+    let a = User::new("alice", Email::new("a@example.com").unwrap(), PasswordHash::from_hash("h".into()));
+    let b = User::new("bob", Email::new("b@example.com").unwrap(), PasswordHash::from_hash("h".into()));
+    assert_ne!(a.id, b.id);
+    assert_eq!(a.username, "alice");
+    assert_eq!(b.username, "bob");
+}