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/types.rs

@@ -656,12 +656,7 @@ impl fmt::Display for Value {
             Value::FieldAccessor(k) => write!(f, ".{}", k.name()),
             Value::Function(f_meta) => write!(f, "<native fn, {:?}>", f_meta.purity),
             Value::Object(o) => {
-                if let Some(pipe) = o
-                    .as_any()
-                    .downcast_ref::<crate::ast::rtl::streams::PipelineNode>()
-                {
-                    write!(f, "PipelineNode[last: {:?}]", pipe.series.get_item(0))
-                } else if let Some(val_series) =
+                if let Some(val_series) =
                     o.as_any()
                         .downcast_ref::<crate::ast::rtl::series::SharedValueSeries>()
                 {