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

crates/domain/tests/value_objects/checksum.rs

@@ -0,0 +1,27 @@
+use domain::value_objects::Checksum;
+
+#[test]
+fn accepts_valid_sha256() {
+    let hex = "a".repeat(64);
+    assert!(Checksum::new(&hex).is_ok());
+}
+
+#[test]
+fn rejects_short() {
+    let hex = "a".repeat(63);
+    assert!(Checksum::new(&hex).is_err());
+}
+
+#[test]
+fn rejects_non_hex() {
+    let mut hex = "a".repeat(63);
+    hex.push('g');
+    assert!(Checksum::new(&hex).is_err());
+}
+
+#[test]
+fn normalizes_to_lowercase() {
+    let hex = "A".repeat(64);
+    let cs = Checksum::new(&hex).unwrap();
+    assert_eq!(cs.as_str(), "a".repeat(64));
+}