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/compiler/bound_nodes.rs

@@ -17,12 +17,19 @@ pub enum BoundKind {
     // Variable Access (Resolved)
     Get(Address),
     
-    // Variable Update (Resolved)
+    // Variable Update (Assignment)
     Set {
         addr: Address,
         value: Box<Node<BoundKind, StaticType>>,
     },
 
+    // Variable Declaration (Local)
+    DefLocal {
+        slot: u32,
+        value: Box<Node<BoundKind, StaticType>>,
+        is_boxed: bool,
+    },
+
     If {
         cond: Box<Node<BoundKind, StaticType>>,
         then_br: Box<Node<BoundKind, StaticType>>,