fix(#2591): validate() rejects out-of-bounds/forward parent index (argus review)

hive-c0re/src/job_queue/tests.rs

@@ -147,6 +147,21 @@ fn unknown_dep_is_rejected_at_submit() {
     assert!(q.submit(spec).is_err());
 }
 
+#[test]
+fn invalid_parent_is_rejected_at_submit() {
+    let q = JobQueue::new(1);
+    let mut spec = rebuild("agent-a", "bad parent");
+    // A forward/out-of-bounds parent index must be refused at validate, not
+    // panic in `insert_group`.
+    spec.nodes = vec![NodeSpec {
+        agent: "agent-a".to_owned(),
+        kind: NodeKind::Reconcile,
+        deps: Vec::new(),
+        parent: Some(3),
+    }];
+    assert!(q.submit(spec).is_err());
+}
+
 // ---- dependency order within a DAG ----
 
 #[test]