fix: apply effective rem to calc function using variables

[Jin-Byun]

Summary

Fix for a bug: Custom effectiveRem value not being applied in Tailwind V4.

• Tailwind V4 uses calc function with rem-based variables (e.g --spacing: 0.25rem) for dynamic values (e.g. w-1 px-17 h-1.5) and more. (e.g. .rounded-4xl { border-radius: calc(var(--radius-xl) * 4})
• Length visitor in the compiler cannot update calc function with rem-based variable directly into px value.
• Update in the Declaration visitor using remToPx transformation similar to Length visitor catches and convert rem-based variables to px-based variables.

Test Plan

• Added test to check whether the effectiveRem set by :root { font-size: 16px; } is applied to border-width: calc(var(--spacing) * 4)
• Tests Passed, typechecked and linted.

[danstepanov]

Preemptive triage note for the 5.0 release, @marklawlor makes the final call.

The bug is real and blocker level, rem inside variable values bypasses the first pass conversion so calc(var(--spacing) * 4) resolves against the default 14. But #336 fixes the same symptom more completely with the inlineRem write back plus the Metro options propagation fix, while this patch misses rem nested inside function tokens. I'd land #336 and close this one, ideally carrying its compiler test over.