This PR migrates packages/react/src/Box/Box.features.stories.tsx away from using styled-components and the sx prop, replacing them with CSS Modules and standard HTML elements.

Changes Made

• Created Box.features.stories.module.css with 7 CSS classes using CSS variables for consistent theming
• Replaced all Box components with standard HTML div elements
• Converted sx prop usage to CSS Module classes with appropriate styling:
  • Border styles using var(--borderWidth-thin) and var(--borderColor-default)
  • Padding using var(--base-size-16)
  • Flexbox layout with flex-grow: 1 for the middle item
  • CSS Grid with grid-template-columns: 1fr 1fr and gap spacing

Before

export const Border = () => (
  <Box sx={{borderWidth: 1, borderStyle: 'solid', borderColor: 'border.default', p: 3}}>
    Box with border
  </Box>
)

After

export const Border = () => <div className={classes.Border}>Box with border</div>

The visual appearance and behavior of all stories remains identical, but the implementation now uses performant CSS Modules instead of runtime-generated styles from styled-components.

All tests pass and Storybook builds successfully with these changes.

Fixes #6567.

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