Update examples to use StreamNode and Series with lookback

The `PipelineNode` has been refactored into `StreamNode`. This commit
updates the example files to reflect this change and also updates the
`series` constructor to accept a lookback parameter.

The `PipelineNode` was an artifact of a previous implementation and is
no longer needed. The `StreamNode` is the appropriate type for
representing the output of a pipe operation.

The `series` function now requires a `lookback` argument to specify the
maximum number of items to store. This makes the behavior of series more
explicit and prevents accidental unbounded memory growth.

src/ast/compiler/bound_nodes.rs

@@ -141,6 +141,7 @@ pub enum BoundKind<T = ()> {
     Again {
         args: Rc<BoundNode<T>>,
     },
+
     Pipe {
         inputs: Vec<Rc<BoundNode<T>>>,
         lambda: Rc<BoundNode<T>>,