Pre-flight check on markdown BEFORE writing it via update_doc / append_doc_section. Returns { ok, errors, warnings, parsed } with parsed counts per format type (imageCount, videoCount, mermaidCount, mathCount, svgCount, calloutCount, crossRefCount, mentionCount, embedCount, detailsCount, headingCount, byteSize, nodeCount, depth) plus structured DocGuardError-equivalent errors (cap breaches) and non-blocking warnings (cross-refs that don't resolve, mention ids that don't resolve, oversize sources, cap-approaching counts). NEVER writes anything; pure parse + analysis. Use when iterating on rich-format markdown to catch problems before burning a write. Cross-ref + mention resolution is gated on caller's accessible workspace set, so unresolved tokens surface in warnings.