Iterate rebalance safety like rqalpha

crates/fidc-core/tests/explicit_order_flow.rs

@@ -631,16 +631,19 @@ fn rebalance_optimizer_prioritizes_higher_target_weight_when_cash_is_tight() {
         )
         .expect("broker execution");
 
-    assert_eq!(
-        portfolio
-            .position("000002.SZ")
-            .map(|position| position.quantity)
-            .unwrap_or(0),
-        900
-    );
+    let lower_weight_qty = portfolio
+        .position("000001.SZ")
+        .map(|position| position.quantity)
+        .unwrap_or(0);
+    let higher_weight_qty = portfolio
+        .position("000002.SZ")
+        .map(|position| position.quantity)
+        .unwrap_or(0);
     assert!(
-        portfolio.position("000001.SZ").is_none(),
-        "higher target weight should consume the limited rebalance cash first"
+        higher_weight_qty > lower_weight_qty,
+        "cash-constrained rebalance should preserve more exposure for the higher target weight, got low={} high={}",
+        lower_weight_qty,
+        higher_weight_qty
     );
     assert!(
         report