Introduce boxing for captured variables

The binder now uses an `UpvalueAnalyzer` to identify local variables
that are captured by nested functions. These identified variables are
marked with `is_boxed: true` during `DefLocal` node creation. The VM
then uses this flag to wrap such variables in a `Value::Cell` (using
`Rc<RefCell<_>>`) to ensure they can be mutated across function calls.
feat: Introduce boxing for captured variables

Add UpvalueAnalyzer to identify variables captured by nested lambdas.
Modify Binder to use the analyzer and mark captured local variables for
boxing.
Update BoundKind::DefLocal to include an `is_boxed` flag.
Update VM to box captured variables when they are defined.
Add tests for upvalue capture detection.

src/main.rs

@@ -220,7 +220,7 @@ impl eframe::App for CompilerApp {
                         }
                     } else {
                         ui.add_enabled_ui(false, |ui| {
-                            ui.button("Save (None loaded)");
+                            let _ = ui.button("Save (None loaded)");
                         });
                     }