🎨 Palette: Add keyboard hints and screen reader support to Mario game

[EiJackGH]

💡 What: Added explicit visual instructions indicating the required keys to play, and added aria-live="polite" to the score container.
🎯 Why: First-time users often don't know the controls for embedded games, leading to frustration. Screen reader users were previously not notified when their score updated.
📸 Before/After: Visually, there is now a semitransparent instructional box in the top-right corner.
♿ Accessibility: Implemented aria-live="polite" and aria-atomic="true" on #score so that dynamic updates are politely announced. Instructional text provides cognitive accessibility for custom key bindings.

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PR created automatically by Jules for task 6837908767097172349 started by @EiJackGH

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