fix(rule-utils): anchor the part- variant regex - #5313

The bug

part-[...] stops working as soon as it is combined with a variant that UnoCSS resolves after it. hover:part-[button]:text-red works, but dark:part-[button]:text-red produces no CSS at all. The utility is silently dropped, with no warning.

Also affected: rtl:, *:, scope-[...]:, group-data-[...]:, @container, and the [&...]: variables variant.

Reproduce

const uno = await createGenerator({ presets: [presetMini({ dark: 'class' })] })

await uno.generate(['hover:part-[button]:text-red'])
// .hover\:part-\[button\]\:text-red::part(button):hover{ ... }

await uno.generate(['dark:part-[button]:text-red'])
// "" (nothing)

Root cause

PartClassesRE in packages-presets/rule-utils/src/pseudo.ts is not anchored, but its handler builds the next matcher with input.slice(match[1].length), which assumes the match starts at index 0. For dark:part-[button]:text-red the regex matches at index 5 and the slice yields ton]:text-red, so no rule ever matches it. The variants that do work only work because they are ordered before variantPartClasses and get consumed first.

Anchoring the regex lets those later variants take their turn, after which part- matches at index 0 as it already does today.

Test

packages-presets/rule-utils/test/pseudo.test.ts gains part classes after another variant: it registers createPartClasses() followed by a dark variant and asserts that dark:part-[button]:foo-1 is matched and emits .dark .dark\:part-\[button\]\:foo-1::part(button){color:foo-1;}. Without the anchor result.matched is an empty set and no CSS is produced.

The existing part classes test is unchanged and still passes, as does the rest of the suite.