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_string_tests.rs

@@ -10,10 +10,7 @@ fn eval(vm: &mut Vm, source: &str) -> Result<JsValue, String> {
 }
 
 fn to_str(vm: &Vm, val: JsValue) -> String {
-    vm.kernel_core()
-        .string_forge()
-        .lookup(val.as_string_index())
-        .unwrap_or_default()
+    vm.lookup_str(val).unwrap_or_default()
 }
 
 fn assert_num_eq(val: JsValue, expected: f64) {