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/optimizer/engine.rs

@@ -439,7 +439,7 @@ impl Optimizer {
                     o_inputs.push(opt);
                 }
                 let o_lambda = self.visit_node(lambda.clone(), sub, path);
-                
+
                 if !inputs_changed && Rc::ptr_eq(&o_lambda, lambda) {
                     return node_rc;
                 }
@@ -453,7 +453,6 @@ impl Optimizer {
                     node.ty.clone(),
                 )
             }
-
             BoundKind::Block { exprs } => {
                 let mut info = UsageInfo::default();
                 if !exprs.is_empty() {