Add now() built-in function

This commit introduces the `now()` built-in function, which returns the
current timestamp in milliseconds. It also includes an integration test
to ensure that `now()` is not constant-folded during AST dumping,
verifying its non-deterministic nature.

src/ast/rtl/datetime.rs

@@ -27,4 +27,14 @@ pub fn register(env: &Environment) {
         }
         Value::Void
     });
+
+    let now_ty = StaticType::Function(Box::new(Signature {
+        params: StaticType::Tuple(vec![]),
+        ret: StaticType::DateTime,
+    }));
+
+    env.register_native("now", now_ty, Purity::SideEffectFree, |_| {
+        let ts = chrono::Utc::now().timestamp_millis();
+        Value::DateTime(ts)
+    });
 }

src/integration_test.rs

@@ -169,6 +169,27 @@ mod tests {
         assert_eq!(format!("{}", env.run_script("NaN").unwrap()), "NaN");
     }
 
+    #[test]
+    fn test_now_function_not_folded() {
+        let env = Environment::new();
+        let source = "(now)";
+        
+        // 1. Check result type and value plausibility
+        let result = env.run_script(source).expect("Failed to run script");
+        if let Value::DateTime(ts) = result {
+            let current = chrono::Utc::now().timestamp_millis();
+            assert!(ts > 0);
+            assert!(ts <= current);
+        } else {
+            panic!("Expected DateTime, got {:?}", result);
+        }
+
+        // 2. Verify it's NOT constant folded in the AST dump
+        let dump = env.dump_ast(source).expect("Failed to dump AST");
+        assert!(dump.contains("Call"), "now() should remain a Call, not a Constant. Dump: \n{}", dump);
+        assert!(!dump.contains("Constant: #"), "now() should NOT be folded into a specific timestamp constant. Dump: \n{}", dump);
+    }
+
     #[test]
     fn test_date_parsing() {
         let env = Environment::new();