feat(example): add Expo uploader example; reorganize examples into example/{react-native,expo}

[Sumasree8]

• Adds a second example app built with Expo, alongside the existing React Native example.
• Groups both under example/: existing app moved to example/react-native/, new app at example/expo/.
• Expo example is a minimal single-screen uploader: pick a video → progress bar → pause/resume. Nothing else.
• Requires a development build (expo run:ios|android), not Expo Go, since the SDK uses native modules.
• Also renames __tests__/ → tests/.