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/ast/environment.rs

@@ -116,11 +116,10 @@ impl Environment {
         }
 
         // 3. Bind & Type Check
-        let mut binder = Binder::new(self.global_names.clone());
-        let mut bound_ast = binder.bind(&untyped_ast)?;
+        let bound_ast = Binder::bind_root(self.global_names.clone(), &untyped_ast)?;
 
         // 4. Optimize
-        bound_ast = TCO::optimize(bound_ast);
+        let bound_ast = TCO::optimize(bound_ast);
 
         // 5. Execute
         let mut vm = VM::new(self.global_values.clone());