fix: preserve arbitrary values using CSS variables during Tailwind v4…

[Yeom-JinHo]

Description

This PR focuses on fixing incorrect arbitrary utility conversions introduced during the Tailwind v4 migration.
Some utilities using CSS variables like gap-[var(--gap)] were mistakenly transformed into gap-(--gap) by the upgrade CLI, causing runtime and style mismatches.
This update restores proper handling for CSS variable–based arbitrary values to ensure accurate conversion and consistent styling.

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Changes

• Early Guard for Complex Arbitrary Values

  • In arbitraryUtilities, when the candidate is functional + arbitrary, conversion is skipped if the raw value contains var(), calc(), spaces, commas, or slashes.
  • Prevents invalid rewrites such as gap-[var(--gap)] → gap-(--gap).

• String Normalization

  • Normalize the raw value to valueStr = String(value) for consistent parsing and formatting.

• Complex Value Handling

  • If isComplex is true, do not attempt bare-value replacements; only emit the bracketed arbitrary form (${root}-[${valueStr}]).

• Modifier Serialization

  • Use printModifier() when emitting classes with modifiers to ensure correct formatting.

• Storage/Signature Access (No Behavior Change)

  • Read precomputed utilities and signatures via designSystem.storage[PRE_COMPUTED_UTILITIES_KEY | UTILITY_SIGNATURE_KEY].get(options.signatureOptions).
  • Read spacing map via designSystem.storage[SPACING_KEY].

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Motivation

The Tailwind v4 migration tool (@tailwindcss/upgrade) introduced incorrect transformations for arbitrary utilities using CSS variables, leading to broken layouts.
This fix ensures the migration process safely ignores complex or variable-based values and preserves valid classnames for reliable build output.

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Breaking Changes

None.
This patch only restores correct handling for arbitrary utility parsing during the migration process.

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Before/After

Before
❌ gap-[var(--gap)] → gap-(--gap) (invalid, broken animation/layout)

After
✅ gap-[var(--gap)] remains intact and functional across builds