fix: reduce code block highlight flicker

Preserve layout with invisible placeholders while worker highlighting loads
Reuse cached highlighted lines on first render when available
Differentiate loading from failed highlighting results for better fallback behavior

packages/ui/src/components/chat/markdown/markdown-worker.ts

@@ -167,6 +167,12 @@ export const highlightLinesInWorker = async (code: string, lang: string): Promis
   return result?.type === 'highlightLines' ? result.lines : null;
 };
 
+/** Return an already-tokenized line result without scheduling a worker request. */
+export const getCachedHighlightedLines = (code: string, lang: string): string[] | null => {
+  const cached = resultCache.get(cacheKeyFor('highlightLines', lang, code));
+  return cached?.type === 'highlightLines' ? cached.lines : null;
+};
+
 /**
  * Tokenize `code` with the given resolved TextMate theme and return per-line
  * styled runs with offsets — for building CodeMirror decorations that match the