fix(unplugin): arithmetic support for defineConsts

[skovhus]

What changed / motivation ?

JS arithmetic on defineVars/defineConsts token references inside stylex.create() silently produced broken CSS (C.a + C.b → "var(--a)var(--b)", C.a - 1 → NaN).

This PR compiles arithmetic to CSS calc() instead — without violating the "transforming a file must not read other files" constraint: the emitted expression is built from the same hash-derived var() strings as today, and defineConsts values are substituted by the existing CSS aggregation step.

• zIndex: C.a + C.b → calc(var(--a) + var(--b)) (→ calc(26 + 14) for consts)
• zIndex: C.elevated - 1 → calc(var(--elevated) - 1)
• marginLeft: -C.gutter → calc(-1 * var(--gutter))
• Works for defineConsts, defineVars (incl. sibling refs), and mixed expressions

Everything that can't map to calc() is now a compile-time error.

Breaking change

⚠️ Comparisons on token refs (e.g. C.a === 'red' ? x : y) previously evaluated silently (string comparison, usually picking the wrong branch) and now fail the build with an actionable error. Null/undefined guards are exempt and keep working.

Linked PR/Issues

Fixes #1597

Additional Context

Screenshots, Tests, Anything Else

Pre-flight checklist

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

[vercel]

@skovhus is attempting to deploy a commit to the Meta Open Source Team on Vercel.

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[skovhus]

@nmn if we don't want to support arithmetic for some reason (would be nice if we could), then at least I think we should fail hard instead of silently emitting broken CSS.

[skovhus]

We just had another production issue due to the underlying issue. @nmn @mellyeliu would love some feedback on the approach on this branch. Happy to put up a PR that fails hard if we don't want arithmetic.

[mellyeliu]

Thanks for fixing this, overall this implementation looks fine and keeps everything per-file. Mind adding some more tests for different arithmetic operations (a + b * c, (a + b) * 2, with brackets, etc) plus a couple of string-concatenation cases like 'translateX(' + token + ')'?

[skovhus]

Another foot gun that I'll need to see if we can solve here:

// $zIndex.dialog is defined as a number

const PRESENTER_Z_INDEX = Number($zIndex.dialog) + 1;

const styles = stylex.create({
  presenter: {
    zIndex: PRESENTER_Z_INDEX, // silently breaks
  },
});