Introduce unified Series trait

This commit introduces the `Series` trait to provide a unified interface
for accessing indexed data across different series types (RecordSeries,
SeriesView, and future SharedSeries). This simplifies the VM's indexer
logic and lays the groundwork for the dual series architecture.

src/ast/rtl/series.rs

@@ -285,6 +285,15 @@ impl Object for RecordSeries {
     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 RecordSeries {
+    fn get_item(&self, index: usize) -> Option<Value> {
+        self.get_record(index)
+    }
 }
 
 // ============================================================================
@@ -328,6 +337,16 @@ impl Object for SeriesView {
     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 SeriesView {
+    fn get_item(&self, index: usize) -> Option<Value> {
+        self.inner.borrow().get_value(index)
+    }
 }
 
 // ============================================================================