fix: ensure explicit nesting selector is always applied

[dummdidumm]

Previously, we were applying an explicit nesting selector to the start of a relative selector chain only when starting the traversal. Prepending the selector is important because it ensures we traverse upwards to the parent rule when the current selectors all matched and there's still more to do. But we forgot to do the prepend for parent rules, which meant that if we were nested two levels deep, we would stop too early. This fix ensures we prepend in that case, too.

Fixes #14178

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[Rich-Harris]

preview: https://svelte-dev-git-preview-svelte-14193-svelte.vercel.app/

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