Keyboard-operable ARIA radiogroup with roving tabindex

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Build a single self-contained page as one HTML file (`index.html`) that renders with no build step and no network calls (inline all CSS and JS, no external fonts or scripts). Build a custom radio group following the WAI-ARIA radiogroup pattern (do NOT use native <input type="radio">). Requirements: - A role="radiogroup" containing exactly three role="radio" options with the ids radio-standard, radio-express, radio-overnight. Exactly one option is checked on load (aria-checked="true"); only the checked option has tabindex="0" and the others have tabindex="-1" (roving tabindex, so the whole group is a single Tab stop). - When a radio has focus, ArrowDown or ArrowRight moves selection to the next option and ArrowUp or ArrowLeft to the previous (wrapping at the ends). Moving selection updates aria-checked (the newly selected option becomes "true", the previous "false"), moves tabindex, and moves DOM focus to the newly selected option. The radio group must be fully operable with the keyboard alone. Use plain, accessible markup.