feat: Add SharedValueSeries and PipelineNode

Introduces `SharedValueSeries` for read-only access to generic `Value`
buffers, and `PipelineNode` which combines an `ObservableStream` with a
`Series` view. This is a foundational step for implementing the `pipe`
operator.

src/ast/rtl/series.rs

@@ -386,6 +386,36 @@ impl<T: ScalarValue> crate::ast::types::Series for SharedSeries<T> {
     }
 }
 
+/// A read-only series for generic Values that shares its buffer with the pipeline.
+#[derive(Clone)]
+pub struct SharedValueSeries {
+    pub buffer: Rc<std::cell::RefCell<RingBuffer<Value>>>,
+}
+
+impl Debug for SharedValueSeries {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        write!(f, "SharedValueSeries[len: {}]", self.buffer.borrow().len())
+    }
+}
+
+impl Object for SharedValueSeries {
+    fn type_name(&self) -> &'static str {
+        "SharedValueSeries"
+    }
+    fn as_any(&self) -> &dyn Any {
+        self
+    }
+    fn as_series(&self) -> Option<&dyn crate::ast::types::Series> {
+        Some(self)
+    }
+}
+
+impl crate::ast::types::Series for SharedValueSeries {
+    fn get_item(&self, index: usize) -> Option<Value> {
+        self.buffer.borrow().get(index).cloned()
+    }
+}
+
 /// A read-only series for Records that shares its buffers with the pipeline.
 #[derive(Clone)]
 pub struct SharedRecordSeries {