Support line-level review progress within a PR diff

Request

Please support line-level review progress within a PR diff.

Reasoning

For large changed files, file-level viewed/unviewed is too coarse. I often review a file in multiple passes and want a way to distinguish the lines I already reviewed from the lines I still need to look at.

Possible behavior

• mark selected lines or ranges as reviewed
• visually de-emphasize already-reviewed lines
• persist the state locally for the current PR / head SHA

A local-only feature would already be useful.

ファイルやテストは指定されていません。まず、PR diff UI における既存のファイル単位の表示済み/未表示状態と、その永続化先を特定してください。実装を完了とみなすには、範囲の選択、レビュー済み行の表示、保存された状態を PR と head SHA に限定することについて、確定した設計が必要です。