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/rtl/series.rs

@@ -5,7 +5,7 @@ use std::rc::Rc;
 use std::sync::Arc;
 
 use crate::ast::environment::Environment;
-use crate::ast::types::{Keyword, Object, RecordLayout, Series, StaticType, Value};
+use crate::ast::types::{Keyword, Object, PushableSeries, RecordLayout, Series, StaticType, Value};
 use crate::ast::types::{Purity, Signature};
 
 // ============================================================================
@@ -144,14 +144,20 @@ impl<T: ScalarValue> Object for ScalarSeries<T> {
     fn into_rc_any(self: Rc<Self>) -> Rc<dyn Any> {
         self
     }
+    fn as_series(&self) -> Option<&dyn Series> {
+        Some(self)
+    }
+    fn as_pushable_series(&self) -> Option<&dyn PushableSeries> {
+        Some(self)
+    }
+}
+
+impl<T: ScalarValue> PushableSeries for ScalarSeries<T> {
     fn push_value(&self, value: Value) {
         if let Some(v) = T::from_value(value) {
             self.data.borrow_mut().push(v);
         }
     }
-    fn as_series(&self) -> Option<&dyn Series> {
-        Some(self)
-    }
 }
 
 impl<T: ScalarValue> Series for ScalarSeries<T> {
@@ -200,12 +206,18 @@ impl Object for ValueSeries {
     fn into_rc_any(self: Rc<Self>) -> Rc<dyn Any> {
         self
     }
-    fn push_value(&self, value: Value) {
-        self.data.borrow_mut().push(value);
-    }
     fn as_series(&self) -> Option<&dyn Series> {
         Some(self)
     }
+    fn as_pushable_series(&self) -> Option<&dyn PushableSeries> {
+        Some(self)
+    }
+}
+
+impl PushableSeries for ValueSeries {
+    fn push_value(&self, value: Value) {
+        self.data.borrow_mut().push(value);
+    }
 }
 
 impl Series for ValueSeries {
@@ -224,10 +236,10 @@ impl Series for ValueSeries {
 // 3. RecordSeries (SoA - Struct of Arrays)
 // ============================================================================
 
-/// Marker supertrait combining `Object` and `Series` for RecordSeries field buffers.
-/// Ensures each field buffer can be stored as a typed series AND used as a dynamic object
-/// (including `push_value` via the `Object` supertrait).
-pub trait SeriesMember: Object + Series {}
+/// Marker supertrait for RecordSeries field buffers.
+/// Ensures each field buffer supports indexed access (Series), dynamic dispatch (Object),
+/// and value insertion (PushableSeries).
+pub trait SeriesMember: PushableSeries {}
 
 impl SeriesMember for ScalarSeries<f64> {}
 impl SeriesMember for ScalarSeries<i64> {}
@@ -321,6 +333,15 @@ impl Object for RecordSeries {
     fn into_rc_any(self: Rc<Self>) -> Rc<dyn Any> {
         self
     }
+    fn as_series(&self) -> Option<&dyn Series> {
+        Some(self)
+    }
+    fn as_pushable_series(&self) -> Option<&dyn PushableSeries> {
+        Some(self)
+    }
+}
+
+impl PushableSeries for RecordSeries {
     fn push_value(&self, value: Value) {
         if let Value::Record(_, values) = value {
             for (i, v) in values.iter().enumerate() {
@@ -332,9 +353,6 @@ impl Object for RecordSeries {
             panic!("push to RecordSeries expects a record");
         }
     }
-    fn as_series(&self) -> Option<&dyn Series> {
-        Some(self)
-    }
 }
 
 impl Series for RecordSeries {
@@ -622,8 +640,11 @@ pub fn register(env: &Environment) {
             let (series_val, val) = (&args[0], &args[1]);
 
             if let Value::Object(obj) = series_val {
-                obj.push_value(val.clone());
-                return Value::Void;
+                if let Some(p) = obj.as_pushable_series() {
+                    p.push_value(val.clone());
+                    return Value::Void;
+                }
+                panic!("push expects a pushable series, got: {}", obj.type_name());
             }
             panic!("push expects a series object as the first argument")
         },