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

crates/domain/tests/value_objects/date_time_stamp.rs

@@ -0,0 +1,30 @@
+use chrono::Utc;
+use domain::value_objects::DateTimeStamp;
+
+#[test]
+fn now_is_recent() {
+    let before = Utc::now();
+    let ts = DateTimeStamp::now();
+    let after = Utc::now();
+    assert!(*ts.as_datetime() >= before);
+    assert!(*ts.as_datetime() <= after);
+}
+
+#[test]
+fn ordering() {
+    let a = DateTimeStamp::from_datetime(
+        chrono::DateTime::parse_from_rfc3339("2024-01-01T00:00:00Z").unwrap().into(),
+    );
+    let b = DateTimeStamp::from_datetime(
+        chrono::DateTime::parse_from_rfc3339("2025-01-01T00:00:00Z").unwrap().into(),
+    );
+    assert!(a < b);
+}
+
+#[test]
+fn serde_roundtrip() {
+    let ts = DateTimeStamp::now();
+    let json = serde_json::to_string(&ts).unwrap();
+    let back: DateTimeStamp = serde_json::from_str(&json).unwrap();
+    assert_eq!(ts, back);
+}