perf: trim observer work and defer Prism during streaming

- Drop characterData from scroll-follow and file-link MutationObservers;
  ResizeObserver already catches content growth.
- extractMermaidBlocks: bail out when markdown has no 'mermaid' at all.
- MarkdownCodeBlock: skip Prism highlighting while code is actively
  changing, render plain pre until settled for 300ms.

packages/ui/src/hooks/useScrollEngine.ts

@@ -159,7 +159,7 @@ export const useScrollEngine = ({
         resize.observe(container);
 
         const mutation = new MutationObserver(onChange);
-        mutation.observe(container, { childList: true, subtree: true, characterData: true });
+        mutation.observe(container, { childList: true, subtree: true });
 
         followObserversRef.current = { resize, mutation };
     }, [containerRef, runFollowBurst]);