refactor: 字符串值改用 JsString 指针表示，键与字面量改用 PermInterner

crates/oxide_vm/tests/builtins_object_tests.rs

@@ -63,11 +63,7 @@ fn object_assign_copies() {
 fn object_assign_boxes_primitive_target() {
     let (_vm, result) = eval("var result = Object.assign('a'); result.valueOf()").unwrap();
     assert!(result.is_string());
-    let rendered = _vm
-        .kernel_core()
-        .string_forge()
-        .lookup(result.as_string_index())
-        .unwrap_or_default();
+    let rendered = _vm.lookup_str(result).unwrap_or_default();
     assert_eq!(rendered, "a");
 }