🎨 Palette: Add role="region" to scrollable areas for screen readers

[lavkushry]

💡 What

Added role="region" alongside the existing aria-label assignments for .visual-figure__body and <pre> elements when they are made horizontally scrollable via JavaScript in theme/visual-edition.js.

🎯 Why

When generic container elements (like <div> or <pre>) are given tabIndex="0" and an aria-label to provide keyboard accessibility for scrolling, screen readers (like VoiceOver or NVDA) will often ignore the label entirely because the element has no semantic meaning. By giving these generic elements an explicit role="region", we tell the screen reader to treat the element as a distinct section of the page, causing it to properly announce the aria-label when the user focuses it.

📸 Before/After

• Before: tabIndex=0 and aria-label on generic elements without a semantic role.
• After: Elements have role="region" to properly identify them as labeled scrollable areas. (No visual changes, purely structural/semantic DOM updates).

♿ Accessibility

Fixes an accessibility anti-pattern where a helpful aria-label meant to assist keyboard users in identifying a scrollable area ("Scrollable diagram region" or "Scrollable Rust code sample") was ignored by assistive technology. Screen readers will now announce these regions successfully.

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PR created automatically by Jules for task 13808635171479012360 started by @lavkushry

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