walnut-markdown-rendering

How Walnut turns model text into rendered output — the two marked instances and the CJK retunes registered on both, why we extend marked through its public API and never patch it, the rich-HTML block chunker's prefix invariant and blank-line content-model rule, DOMPurify policy per surface, and the marked 15 → 18 upgrade path. MUST read before changing anything in web/src/utils/markdown.ts, rich-blocks.ts, or adding…