Refactor PushableSeries trait

Introduce a new trait `PushableSeries` to distinguish objects that can
have values pushed into them from those that are only readable series.
This clarifies the API and allows for more precise type checking.

The `Object` trait no longer has a default `push_value` implementation.
Instead, `PushableSeries` now provides this functionality.
`SeriesMember` has also been updated to require `PushableSeries` for
RecordSeries field buffers.

src/ast/types.rs

@@ -176,7 +176,7 @@ impl RecordLayout {
     }
 }
 
-/// Interface for custom objects (Closures, Series, Streams)
+/// Interface for custom objects (Series, Streams, SyntaxNodes, and custom extensions).
 pub trait Object: fmt::Debug {
     fn type_name(&self) -> &'static str;
     fn as_any(&self) -> &dyn Any;
@@ -185,17 +185,21 @@ pub trait Object: fmt::Debug {
     /// `Rc<T>` via `Rc::downcast::<T>()`. Requires `arbitrary_self_types` (stable since 1.81).
     fn into_rc_any(self: Rc<Self>) -> Rc<dyn Any>;
 
-    /// Pushes a value into this object if it is a mutable series.
-    /// The default implementation panics — only series types override this.
-    fn push_value(&self, _val: Value) {
-        panic!("{} does not support push_value", self.type_name());
-    }
-
-    /// Optional optimization: If this object behaves like a time series (indexable lookbacks),
-    /// it can return a reference to its Series trait implementation.
+    /// If this object supports indexed lookback access, returns a `Series` reference.
     fn as_series(&self) -> Option<&dyn Series> {
         None
     }
+
+    /// If this object supports pushing new values, returns a `PushableSeries` reference.
+    fn as_pushable_series(&self) -> Option<&dyn PushableSeries> {
+        None
+    }
+}
+
+/// Extension of `Object + Series` for types that also accept new values via `push_value`.
+/// Only series types that are mutable (ScalarSeries, ValueSeries, RecordSeries) implement this.
+pub trait PushableSeries: Object + Series {
+    fn push_value(&self, val: Value);
 }
 
 /// Unified interface for all series types (Local RecordSeries, SeriesViews, and future SharedSeries).