Move large example public assets to remotion.media

- Delete 3 unreferenced files from packages/example/public/ (~14 MB):
  long-video.mp4, spiral_.mp4, video.mp4.
- Move 12 largest referenced fixtures and sounds/1.wav-9.wav to
  https://remotion.media/... URLs (~157 MB removed from public/).
- Inline the remote URLs at the call sites in packages/example/src/.
- Add a small cached-example-media module in @remotion/example-videos
  that downloads the files into node_modules/.videos as part of make,
  so media-parser tests still see them as local file paths.

Co-authored-by: Cursor <cursoragent@cursor.com>