Address soundness hole in cycle prevention

tests/execution/examples.rs

@@ -9,9 +9,9 @@ use futures::FutureExt;
 async fn test_example_a_fan_out() {
     let dag = DagRunner::new();
 
-    let base = dag.add_task(Seed::new(10)); // Input=() → no depends_on, returns TaskHandle
-    let plus1 = dag.add_task(Inc::new()).depends_on(&base); // 11
-    let plus2 = dag.add_task(Inc::new()).depends_on(&base); // 11
+    let base: TaskHandle<_> = dag.add_task(Seed::new(10)).into(); // Input=() → no depends_on, returns TaskHandle
+    let plus1 = dag.add_task(Inc::new()).depends_on(base); // 11
+    let plus2 = dag.add_task(Inc::new()).depends_on(base); // 11
 
     dag.run(|fut| tokio::spawn(fut).map(Result::unwrap))
         .await
@@ -29,7 +29,7 @@ async fn test_example_b_fan_in() {
     let n = dag.add_task(task_fn(|_: ()| async { 42usize }));
     let f = dag.add_task(task_fn(|_: ()| async { true }));
 
-    let output = dag.add_task(OutputResult::new()).depends_on((&s, &n, &f));
+    let output = dag.add_task(OutputResult::new()).depends_on((s, n, f));
 
     dag.run(|fut| tokio::spawn(fut).map(Result::unwrap))
         .await
@@ -42,11 +42,11 @@ async fn test_example_c_many_to_many() {
     let dag = DagRunner::new();
 
     let x = dag.add_task(Seed::new(2));
-    let y = dag.add_task(Seed::new(3));
+    let y = dag.add_task(Seed::new(3)).into();
     let z = dag.add_task(Seed::new(5));
 
-    let sum_xy = dag.add_task(Add::new()).depends_on((&x, &y)); // 5
-    let prod_yz = dag.add_task(Mul::new()).depends_on((&y, &z)); // 15
+    let sum_xy = dag.add_task(Add::new()).depends_on((x, &y)); // 5
+    let prod_yz = dag.add_task(Mul::new()).depends_on((&y, z)); // 15
     let total = dag.add_task(Add::new()).depends_on((&sum_xy, &prod_yz)); // 20
 
     dag.run(|fut| tokio::spawn(fut).map(Result::unwrap))
@@ -59,12 +59,14 @@ async fn test_example_c_many_to_many() {
 async fn test_single_input_forms() {
     let dag = DagRunner::new();
 
-    let s = dag.add_task(task_fn(|_: ()| async { "hello".to_owned() }));
+    let s = dag
+        .add_task(task_fn(|_: ()| async { "hello".to_owned() }))
+        .into();
 
     // Preferred form: &node
     let upper1 = dag
         .add_task(task_fn(|s: String| async move { s.to_uppercase() }))
-        .depends_on(&s);
+        .depends_on(s);
 
     // Also allowed: (&node,)
     let upper2 = dag