Promote CSS-length converter to #v0/utilities#toUnit

[johnleider]

Problem

PR #201 adds a padding prop to Carousel.Root that accepts number | string and converts numbers (and numeric strings) to px. The conversion is inlined in CarouselRoot.vue:

padding: toRef(() => {
  if (isNullOrUndefined(padding)) return undefined
  if (isNumber(padding)) return `${padding}px`
  return /^-?\d+(\.\d+)?$/.test(padding) ? `${padding}px` : padding
}),

This is the first CSS-length prop in v0. The next one (Dialog width, Popover offset, Slider thumbSize, Image size, etc.) will copy-paste and drift on edge cases (empty string, whitespace, scientific notation, NaN).

Proposal

Add toUnit(value, unit = 'px') to #v0/utilities when a second caller lands. Signature and semantics roughly matching Vuetify 3's convertToUnit, named toUnit to match the to* prefix convention (PHILOSOPHY §3.3):

export function toUnit (value: number | string | null | undefined, unit = 'px'): string | undefined {
  if (isNullOrUndefined(value) || value === '') return undefined
  if (isNumber(value)) return `${value}${unit}`
  return /^-?\d+(\.\d+)?$/.test(value) ? `${value}${unit}` : value
}

Tree-shakeable, #__NO_SIDE_EFFECTS__, fits alongside clamp / range.

Open questions

Decide before promoting:

• "" → undefined or ""?
• "0" → "0px" or passthrough?
• " 10 " (whitespace) — trim or fail?
• "1e3" — convert or passthrough?
• Separate toUnit vs. generic toCssValue — does a consumer ever need units other than px?

Pick semantics explicitly; don't inherit by accident.

Acceptance

• Helper lives at packages/0/src/utilities/helpers.ts with tests covering every edge case listed above.
• CarouselRoot.vue padding: block replaced with toUnit(padding).
• At least one additional caller wired up in the same PR (otherwise the "2+ callers" bar isn't met and this stays inline).

Context

• Grep anchor in source: TODO(utilities): promote to \toUnit`inpackages/0/src/components/Carousel/CarouselRoot.vue`.
• PHILOSOPHY §2.7 — utilities must be pure, tree-shakeable, and justify their existence.
• Related PR: fix(Carousel): auto-mirror viewport padding to scroll-padding #201.

[johnleider]

Closing — not needed. PR #201 was refactored to drop the inline CSS-length converter entirely. The viewport now auto-mirrors consumer-set padding onto scroll-padding via ResizeObserver + getComputedStyle, so v0 never takes a CSS-length value as a prop. The toUnit helper this issue proposed is no longer motivated by any source call site; we can revisit if a real dimension-style prop ever lands (note: still unlikely given the headless contract in PHILOSOPHY §1.1 / §5.3).