MarkdownDocument::block runs a BlockCommand at the caret: heading
levels, paragraph, promote and demote; quote and list toggles;
rule, code, math, toc, footnote, link definition and front matter
inserts; empty paragraphs before and after; delete. Formats and
block commands share one runner off the UI thread; FormatRefusal
is now EditRefusal.
Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>
