[codemods] Convert static css props to StyleX (Emotion→StyleX, 2/6)

[ahmedsadid]

What changed / motivation ?

Part of adding a tool that migrates Emotion styles to StyleX (#1766). This PR
does its first real conversion: a static Emotion css prop becomes a
stylex.create block plus a stylex.props(...) reference.

// before
<div css={{ color: 'red' }}>Badge</div>

// after
const styles = stylex.create({ badge: { color: 'red' } });
<div {...stylex.props(styles.badge)}>Badge</div>

• Covers the object forms with static values: css={{ … }} and css({ … }).
• Converts a file only when it can safely convert everything in it. If a
  file has anything it can't confidently handle yet — :hover/media conditions,
  a value like '8px 16px', a template literal, css on a custom component, or
  a file that already uses StyleX — it leaves the file unchanged and reports why.
  (Converting just the safe parts of a mixed file comes in a later PR.)
• Written in two layers: general code that generates StyleX (src/core/) and
  Emotion-specific code that reads the styles and edits the JSX
  (src/adapters/emotion/). The general layer knows nothing about Emotion, so it
  can be reused for other source libraries later.

Testing

• Input → output fixtures, each also verified to compile, lint cleanly, and
  render the same CSS as the Emotion original.
• Covers conversions (single prop, css(...) call form, two components in one
  file) and refusals (pseudo-selectors, shorthand conflict, template literal,
  custom component, file already using StyleX).
• The general layer is unit-tested with styles fed in directly (no Emotion),
  confirming it's self-contained.

Linked PR/Issues

Part of #1766 · builds on #1768

Additional Context

Stacked on #1768 (the package scaffold), but this change stands on its own to
review.

Pre-flight checklist

• I have read the contributing guidelines
• Performed a self-review of my code

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