🎨 Palette: Add keyboard instructions and accessible score to Mario Game

[EiJackGH]

💡 What: Added visible keyboard instructions overlay to the Mario Game and improved the accessibility of the score counter by wrapping it in an aria-live region.
🎯 Why: Users who aren't familiar with standard web game conventions might not know how to jump (Space or Up Arrow). Furthermore, screen readers wouldn't automatically announce score updates.
♿ Accessibility: Separated the dynamic score value into a span with aria-live="polite" and aria-atomic="true" so screen readers will read the score naturally without repeating the static "Score:" label. The instructions overlay uses pointer-events: none to prevent it from blocking underlying click interactions.

---

PR created automatically by Jules for task 11992795797682397270 started by @EiJackGH

[google-labs-jules]

👋 Jules, reporting for duty! I'm here to lend a hand with this pull request.

When you start a review, I'll add a 👀 emoji to each comment to let you know I've read it. I'll focus on feedback directed at me and will do my best to stay out of conversations between you and other bots or reviewers to keep the noise down.

I'll push a commit with your requested changes shortly after. Please note there might be a delay between these steps, but rest assured I'm on the job!

For more direct control, you can switch me to Reactive Mode. When this mode is on, I will only act on comments where you specifically mention me with @jules. You can find this option in the Pull Request section of your global Jules UI settings. You can always switch back!

New to Jules? Learn more at jules.google/docs.

---

For security, I will only act on instructions from the user who triggered this task.