implement WithLookup

fix/src/runtime/vm.rs

@@ -891,8 +891,60 @@ impl Runtime {
                 root.with_scope = scope.prev;
             }),
             WithLookup => {
-                let _name = self.read_string_id();
-                todo!("implement WithLookup (force with_scope)");
+                let name = self.read_string_id();
+                let mut depth = 0;
+
+                loop {
+                    let found_scope = self.arena.mutate_root(|_, root| {
+                        let mut cur = root.with_scope;
+                        for _ in 0..depth {
+                            if let Some(s) = cur {
+                                cur = s.prev;
+                            } else {
+                                break;
+                            }
+                        }
+
+                        if let Some(scope) = cur {
+                            root.push_stack(scope.env);
+                            true
+                        } else {
+                            false
+                        }
+                    });
+
+                    if !found_scope {
+                        let name_str = self.strings.resolve(name.0).unwrap_or("«unknown»");
+                        return Runtime::handle_vm_error(
+                            self,
+                            vm_err(format!("undefined variable '{name_str}'")),
+                        );
+                    }
+
+                    if let err @ Action::Done(Err(_)) = self.force_tos() {
+                        return err;
+                    }
+
+                    let lookup_result = self.arena.mutate_root(|_, root| {
+                        let val = root.pop_stack();
+                        let Some(attrs) = val.as_gc::<AttrSet<'_>>() else {
+                            return Err(vm_err("value in 'with' scope must be a set"));
+                        };
+
+                        if let Some(v) = attrs.lookup(name) {
+                            root.push_stack(v);
+                            Ok(true) // Found it
+                        } else {
+                            Ok(false) // Not in this scope, try the next one
+                        }
+                    });
+
+                    match lookup_result {
+                        Ok(true) => break, // Successfully resolved and pushed to stack
+                        Ok(false) => depth += 1, // Move to the parent 'with' scope
+                        Err(e) => return Runtime::handle_vm_error(self, e),
+                    }
+                }
             }
 
             LoadBuiltins => {