Add active state support for TOC links in MetaBar

### Enter your suggestions in details: ## Summary The `MetaBar` component renders a Table of Contents (TOC) using heading items, but currently has no way to visually indicate which heading is active (i.e., currently visible in the viewport during scroll). ## Proposed Changes 1. Add an `activeSlug` prop to `MetaBar` (or an `isActive` prop on the link component) so consumers can indicate which heading is currently active. 2. Add a CSS class (e.g. `.tocLink` / `.tocLinkActive`) in `index.module.css` that styles the active link using `box-shadow` instead of `border-left` to avoid horizontal layout shift: ```css .tocLink { box-shadow: inset 2px 0 0 transparent; padding-left: 0.25rem; } .tocLinkActive { box-shadow: inset 2px 0 0 var(--color-primary); font-weight: 600; color: var(--color-primary); } ``` Using box-shadow instead of border-left ensures the layout doesn't shift when the active item changes during scrolling. Related - nodejs/doc-kit#657 ---