feat(loading): support typewriter story teaser during first scene generation

app/play/page.tsx

@@ -402,6 +402,7 @@ function PlayInner() {
   const [error, setError] = useState<string | null>(null);
   const [presentation, setPresentation] = useState(false);
   const [lastExitLabel, setLastExitLabel] = useState<string | null>(null);
+  const [teaserText, setTeaserText] = useState<string | null>(null);
 
   const startedRef = useRef(false);
   const poolRef = useRef<Map<string, PrefetchEntry>>(new Map());
@@ -694,6 +695,25 @@ function PlayInner() {
       return;
     }
 
+    if (livePayload) {
+      fetch("/api/teaser", {
+        method: "POST",
+        headers: {
+          "Content-Type": "application/json",
+          ...getByoHeaders(),
+        },
+        body: JSON.stringify({ worldSetting: livePayload.worldSetting }),
+      })
+        .then(async (r) => {
+          if (!r.ok) return;
+          const data = await r.json();
+          if (data.teaser) {
+            setTeaserText(data.teaser);
+          }
+        })
+        .catch(() => {});
+    }
+
     type PrebakedFirstAct = StartResponse & {
       worldSetting: string;
       styleGuide: string;
@@ -1155,6 +1175,7 @@ function PlayInner() {
           onAdvance={onAdvance}
           onSelectChoice={onSelectChoice}
           fullViewport
+          teaserText={teaserText}
         />
       </div>
     );
@@ -1194,6 +1215,7 @@ function PlayInner() {
           onBackgroundClick={onBackgroundClick}
           onAdvance={onAdvance}
           onSelectChoice={onSelectChoice}
+          teaserText={teaserText}
           aboveCanvas={
             <button
               type="button"