Upgrade React Native Fixture App

Overview

The fixture app lives in fixture/react-native/. It is the primary test vehicle for FlashList on iOS and Android. The web fixture (fixture/web/) uses Expo and is independent — check it builds after the upgrade but it does not need the same dependency changes.

Step 1 — Research the Target Version

1. Check the latest stable RN version:

   npm view react-native@latest version