🎨 Palette: Improve Mario game accessibility and keyboard UX

[EiJackGH]

💡 What: Added visible keyboard instructions ("Press Space or Up Arrow to jump") and wrapped the score value in an aria-live region.
🎯 Why: Users without explicit instruction might not know how to interact with the game. Screen readers previously would read the entire string "Score: N" redundantly instead of just the number change.
📸 Before/After: Added helper text underneath the score explicitly detailing jump controls.
♿ Accessibility: Separated dynamic state into a specific span with aria-live="polite" and aria-atomic="true" for cleaner screen reader feedback.

---

PR created automatically by Jules for task 7006283652904807895 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.