Skip Prism syntax highlighting while tool call is streaming — use plain
<pre> during streaming and only run Prism once at completion. Also throttle
scrollIntoView to once per 150ms to avoid layout thrashing.
Profiled with Playwright + CDP: for 200-cell diagrams the longest
browser task dropped from 6.7s to ~450ms and total long-task time
fell from ~10.9s to ~1.1s.