fix(streamdown): use useDeferredValue for blocks-state, prevent React #185 cascade

[cristicretu]

We hit React #185 with Streamdown in production at Anara. About 15 events/hr at peak across thousands of users. Bumping experimental_throttle to 100ms (per #140) closed roughly 86% of those, but the rest needed a code change.

The cascade sits in index.tsx:

const [displayBlocks, setDisplayBlocks] = useState<string[]>(blocks);
useEffect(() => {
  if (mode === "streaming" && !animatePlugin) {
    startTransition(() => setDisplayBlocks(blocks));
  } else {
    setDisplayBlocks(blocks);
  }
}, [blocks, mode]);

Each streaming token gives blocks a new ref, the effect fires, setDisplayBlocks queues an update. startTransition lowers the priority but doesn't drop intermediates, so under SSE bursts you stack 50+ updates inside one commit and trip React's nested-update guard.

useDeferredValue does the same job without the setStates:

const deferredBlocks = useDeferredValue(blocks);
const blocksToRender =
  mode === "streaming" && !animatePlugin ? deferredBlocks : blocks;

First render returns blocks directly so SSR hydration is unchanged. The animatePlugin path stays synchronous because the plugin needs the freshest content per render.

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