Add into_rc_any and push_value to Object

This commit introduces the `into_rc_any` and `push_value` methods to the
`Object` trait.

The `into_rc_any` method allows for efficient, zero-cost downcasting of
`Rc<Self>` to `Rc<dyn Any>`. This is a crucial step for enabling more
flexible dynamic dispatch and type manipulation within the VM.

The `push_value` method is implemented for series types (`ScalarSeries`,
`ValueSeries`, `RecordSeries`) to allow pushing values directly onto
them. This simplifies the `push` intrinsic function, removing the need
for manual downcasting within its implementation. The default
implementation for non-series objects panics, enforcing that only
mutable series support this operation.

Additionally, `ScalarValue` now includes a `from_value` method to
facilitate converting `Value` back into concrete scalar types. This
improves type safety and reduces boilerplate code when handling scalar
conversions.

src/ast/types.rs

@@ -180,6 +180,16 @@ pub trait Object: fmt::Debug {
     fn type_name(&self) -> &'static str;
     fn as_any(&self) -> &dyn Any;
 
+    /// Converts `Rc<Self>` into `Rc<dyn Any>`, enabling zero-copy downcasting to a concrete
+    /// `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.
     fn as_series(&self) -> Option<&dyn Series> {