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/rtl/prelude.myc

@@ -7,3 +7,25 @@
              (do ~body (again))
            )))
 )
+
+;; Creates a stateful cache (Series) from a stateless stream.
+;; 
+;; Pipes are highly optimized, stateless stream-routers that do not allocate memory 
+;; for historical data. If multiple indicators need to read historical data from 
+;; a stream, or if you need to access previous items via index (e.g. `(my_data 5)`), 
+;; you must explicitly cache the stream.
+;;
+;; Parameters:
+;;   lookback: The maximum number of items to keep in memory (e.g. 100).
+;;   type:     The data type or schema of the series (e.g. :float or {:price :float}).
+;;   src:      The source stream to observe and cache.
+;;
+;; Returns:
+;;   A stateful Series object that automatically updates when the source stream fires.
+(macro cache [lookback type src]
+  `(do
+     (def data (series ~lookback ~type))
+     (pipe [~src] (fn [s] (do (push data s))))
+     data
+   )
+)