`@remotion/example`: Add Rive to experimental controls showcase + effects testbed

- Add a `RemotionRiveCanvas` tile to `ExperimentalControlsShowcase` so we can
  verify schema + sequenceVisualStyleSchema controls (translate / scale /
  rotate / opacity / hidden) wire through.
- Add `rive-effects-testbed` composition (`packages/example/src/Rive/RiveEffects.tsx`)
  that mirrors `EffectsTestbed` but uses `<RemotionRiveCanvas>` to exercise
  the effects pipeline (baseline, tint, halftone, animated blur, animated
  wave, and a stacked chain). The animated blur tile also covers the
  "redraw on effect change" path: its radius pulses between frames.

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