fix: 资料流提早显现内容,避免空白缺口被误认为到底

Reveal 加 300px 前置 viewport margin,内容在进入视口前淡入;
无限滚动预加载触发线 200px → 1000px,下一页提前请求。

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/components/messageStream/MessageStream.tsx

@@ -62,7 +62,9 @@ export function MessageStream({ scope }: MessageStreamProps) {
           }
         }
       },
-      { rootMargin: "200px" },
+      // Prefetch the next page well before the sentinel is visible so content
+      // is already in place as the user scrolls — no blank gap at the bottom.
+      { rootMargin: "1000px" },
     );
     io.observe(el);
     return () => io.disconnect();