codemods] Partial conversion — flag unsupported spots, merge into existing StyleX (Emotion→StyleX, 5/6)

[ahmedsadid]

What changed / motivation ?

Part of migrating Emotion styles to StyleX (#1766). Until now the codemod was
all-or-nothing per file: one thing it couldn't handle and it left the whole file
untouched. That's too conservative for real code, where a file usually has a few
tricky spots among many easy ones. This PR makes it convert what it safely can
and leave the rest clearly marked.

// before (one file, mixed)
<span css={{ color: 'gray' }}>convert me</span>
<button css={{ ':focus': {...}, ':hover': {...} }}>flag me</button>

// after
const styles = stylex.create({ panel: { color: 'gray' } });

<span {...stylex.props(styles.panel)}>convert me</span>
{/* TODO(stylex-migration): :focus/:hover priority disagreement */}
<button css={{ ':focus': {...}, ':hover': {...} }}>flag me</button>

• Converts every site it can and drops a TODO(stylex-migration: …) comment,
  with the reason, on each site it can't — instead of skipping the whole file.
• Files that already use StyleX are no longer skipped: new styles merge into the
  existing stylex.create({ … }) block, adding keys without touching the old
  ones. Only the code it adds gets reformatted.
• Still refuses a whole file when it can't safely read it at all — e.g. the file
  uses StyleX through named imports (import { create }) rather than the
  namespace form (import * as stylex) the tool expects.

Testing

• A mixed-file fixture where some sites convert and one gets a TODO — the
  converted output still compiles, passes lint, and renders the same CSS.
• A merge fixture: new styles are added to a file's existing stylex.create
  without changing its existing keys.
• The previous whole-file "skip" fixtures are now "flag" fixtures — each
  unsupported case (priority conflict, shorthand conflict, template literal,
  custom-component css, cross-element selector, order-dependent media) leaves a
  TODO and converts everything around it.
• A refuse fixture: a file using StyleX via named imports is left unchanged.

Linked PR/Issues

Part of #1766 · builds on #1771

Additional Context

Stacked on #1771, but 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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