Document artifacts give every block a stable id so comment threads stay pinned

The doc artifact template gained an anchors script that assigns a short derived id to every commentable block: headings, paragraphs, list items, and table cells. It preserves author-served ids, removes duplicates created by paste or split operations, re-runs under a MutationObserver to catch later edits, and strips duplicate ids on save. The skill instructions now tell Claude to preserve existing block ids when editing a document, warning that renaming or dropping one detaches the comment thread pinned to that block.