🎨 Palette: Add accessibility and instructions to Mario game

[EiJackGH]

What: Added visible instructional text for game controls ("Press Space or Up Arrow to jump") and ARIA attributes (aria-live="polite", aria-atomic="true") to the score element in mario-game.njk.
Why: First-time players don't naturally know the key bindings for custom HTML5 games, and screen readers need explicit ARIA tags to announce dynamic score updates effectively.
Before/After: The game overlay now contains a helpful semi-transparent "Press Space or Up Arrow to jump" message below the score.
Accessibility: The score element will now dynamically announce changes to screen readers, greatly improving the playability and accessibility of the game for visually impaired users.

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PR created automatically by Jules for task 15211173977148444519 started by @EiJackGH

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