[raiseCatError]

• Engine (src/markdown/diagnostics.ts): Markdown (missing files, bad #fragments, missing images and alt text, undefined references/footnotes, unused footnotes, repeated headings), HTML (missing assets, img alt, form labels, duplicate ids, unresolved fragments, unclosed/unexpected tags; templated attributes unverified), CSS (url() assets, unbalanced braces). Severities error / warning / info / unverified; external URLs only counted, never fetched.
• Safety: offline; no script/style/command run; lstat-only checks confined to the project root by lexical resolution + realpath (symlink escapes reported as not checked); absolute paths never probed; bounded checks/findings/bytes; nothing written.
• Runner: off until Problems is opened; follows the text with a debounce, aborts in-flight runs, drops results for an older revision, disposed with the editor.
• Surface: F1 e (Markdown), palette (HTML), F1 (CSS), P in read-only views and the split preview; search, Tab filter, Enter jumps to and selects the exact text, Del dismiss, Ctrl+R refresh; works in source, preview, hybrid, split, edit; counts in the status row.

Tests: diagnostics.test.ts (12), problems.test.ts (8); combined run with workspace/hybrid/editor/Ask-audit suites: 426 pass. Independent review: running. Not run: full suite, physical QA.
Refs #466 (browser console remains a separate slice).