domain: add cross-cutting value objects (SystemId, DateTimeStamp, Checksum, StructuredData)

crates/domain/tests/value_objects/system_id.rs

@@ -0,0 +1,31 @@
+use domain::value_objects::SystemId;
+use uuid::Uuid;
+
+#[test]
+fn unique_generation() {
+    let a = SystemId::new();
+    let b = SystemId::new();
+    assert_ne!(a, b);
+}
+
+#[test]
+fn uuid_roundtrip() {
+    let uuid = Uuid::new_v4();
+    let id = SystemId::from_uuid(uuid);
+    assert_eq!(*id.as_uuid(), uuid);
+}
+
+#[test]
+fn display_matches_uuid() {
+    let uuid = Uuid::new_v4();
+    let id = SystemId::from_uuid(uuid);
+    assert_eq!(id.to_string(), uuid.to_string());
+}
+
+#[test]
+fn serde_roundtrip() {
+    let id = SystemId::new();
+    let json = serde_json::to_string(&id).unwrap();
+    let back: SystemId = serde_json::from_str(&json).unwrap();
+    assert_eq!(id, back);
+}