Add pipe-buffered function

Implements `pipe-buffered`, a new primitive that allows accumulating
values from multiple input streams into internal series buffers before
executing a user-provided lambda. This is useful for implementing
lookback logic and ensuring that the lambda only executes when
sufficient data is available in all input streams.

The function takes a `lookback` integer, input streams, and a lambda.
The lambda receives `Series` objects for each input, allowing indexed
access to past values. The `pipe-buffered` function manages the
buffering and fill-gate logic.

tests/pipeline.rs

@@ -200,9 +200,10 @@ fn test_pipe_buffered_lambda_params_are_series() {
     )";
     let dump = env.dump_ast(source).expect("dump_ast failed");
 
-    // Lambda param should be Series<Record>, not just Record or Any
+    // Lambda param should be Series<Record>, not just Record or Any.
+    // The AST dump uses Debug format: "Series(Record(...))"
     assert!(
-        dump.contains("series<"),
+        dump.contains("Series(Record("),
         "Lambda param should be typed as Series, not raw value.\nDump:\n{dump}"
     );
 }