refactor: make DagRunner interface single-threaded

docs/COMPARISONS.md

@@ -15,7 +15,7 @@ impl Value { async fn run(&self) -> i32 { self.0 } }
 #[task]
 impl Add { async fn run(a: &i32, b: &i32) -> i32 { a + b } }
 
-let dag = DagRunner::new();
+let mut dag = DagRunner::new();
 let x = dag.add_task(Value(2));
 let y = dag.add_task(Value(3));
 let sum = dag.add_task(Add).depends_on((x, y));

docs/CYCLE_PREVENTION.md

@@ -8,7 +8,7 @@ Most DAG libraries check for cycles when you execute the graph:
 
 ```rust
 // Other libraries: cycle detected at RUNTIME
-let dag = DagRunner::new();
+let mut dag = DagRunner::new();
 let a = dag.add_task(TaskA);
 let b = dag.add_task(TaskB);
 a.depends_on(b);
@@ -20,7 +20,7 @@ dag.run(|fut| async move { tokio::spawn(fut).await.unwrap() })
 
 ```rust
 // dagx: cycle prevented at COMPILE TIME
-let dag = DagRunner::new();
+let mut dag = DagRunner::new();
 let a_builder = dag.add_task(TaskA);
 let b_builder = dag.add_task(TaskB);