[Feature request] Split and refine the Mobile accessibility checklist

### Description

The first version of the Mobile accessibility checklist is now live in the [repository](https://github.com/github/annotation-toolkit/blob/main/checklists/mobile-checklist.md) and on the [primer.style docs site](https://primer.style/accessibility/tools-and-resources/checklists/mobile-checklist/) (shipped via #41, closing #36).

In practice, the mobile checklist ended up about twice as long as the existing web checklists. That length makes it harder to scan and harder to hand to a single audience without a lot of "skip this section" caveats. This issue tracks the follow-up edits and improvements needed to bring it in line with the rest of the toolkit.

### Requested change

Refactor the Mobile checklist into multiple, more focused variants, mirroring how the web checklists are split today:

- **Split by audience:** Separate the content into a mobile **designer** checklist and a mobile **engineer** checklist, so each role only sees items relevant to their phase of work.
- **Split by depth:** Offer a **full** checklist (the comprehensive list we have now) and a **high-impact** short list that surfaces the most critical items for teams who can't run through the full thing on every project.
- **Rebuild the Figma component:** Update the Checkpoints component in the Figma library so the new variants are selectable as properties, consistent with the existing web checklist component. This is a heavier lift than the markdown work since the current

checklists/mobile-checklist.md から始め、既存の Web チェックリストと比較して、designer、engineer、full、high-impact の各バリアントを定義します。次に、Figma ライブラリの Checkpoints コンポーネントと #41 で提供された作業を確認します。完了とは、Markdown のバリアントがセクションをスキップする注意書きなしで使用でき、Figma コンポーネントが新しいバリアントを選択可能なプロパティとして公開している状態です。

索引モデルが issue の本文から書いたものです。