feat(onboarding): first-chart reveal becomes a star convergence, no buttons (F4)

TASK-birth-sky-followup-20260928 F4:
- pure timeline / seeded scatter / interpolation in lib/birth-sky/converge.ts;
  drawBirthSky takes an optional frame (positions interpolated, alphas
  multiplied) so the settled frame is the exported cover call for call
- lazy full-screen <dialog> overlay driven by requestAnimationFrame at
  viewport x DPR; tap / click / Esc end it; reduced motion only fades
- data, serif and overlay chunk must all arrive inside the 3 s budget,
  otherwise skipped silently; page.tsx wiring unchanged
- one sonner hint after it ends; dialog reveal variant (继续, is-reveal) removed
- DESIGN §17, VOICE, CHANGELOG, checklist, PROGRESS with verification

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01N4f2nya58RoRu4yEmJgRGE

frontend/src/hooks/use-birth-sky-reveal.ts

@@ -1,34 +1,49 @@
 "use client";
 
 import { useCallback, useEffect, useRef, useState } from "react";
+import { toast } from "sonner";
 
-import { loadBirthSkyCover, releaseBirthSkyCover, type BirthSkyCover } from "@/lib/birth-sky/cover";
-import { startBirthSkyReveal } from "@/lib/birth-sky/reveal";
+import { loadBirthSkyRevealSky, startBirthSkyReveal, type BirthSkyRevealSky } from "@/lib/birth-sky/reveal";
+
+/** Shown once on the home page after the convergence ends (VOICE.md). */
+export const BIRTH_SKY_REVEAL_HINT = "这片天空在星盘页可以保存。";
 
 /**
- * The one-time 「那一刻的天空」 reveal after the reader's own first chart
- * (TASK-birth-sky-cover-20260928 T6). Owns its state so Home only wires it:
- * `begin` goes to the onboarding hook, `cover` / `close` to the dialog.
- * Saving one's own birth place happens once per account, so no "seen" flag.
+ * The one-time 「那一刻的天空」 convergence after the reader's own first chart
+ * (TASK-birth-sky-cover-20260928 T6 → TASK-birth-sky-followup-20260928 F4).
+ * Owns its state so Home only wires it: `begin` goes to the onboarding hook,
+ * `cover` / `close` to the overlay. `cover` is the sky plus the serif verdict,
+ * ready before the first frame. Saving one's own birth place happens once per
+ * account, so no "seen" flag.
  */
 export function useBirthSkyReveal() {
-  const [cover, setCover] = useState<BirthSkyCover | null>(null);
+  const [cover, setCover] = useState<BirthSkyRevealSky | null>(null);
   const cancel = useRef<(() => void) | null>(null);
+  const playing = useRef(false);
 
   const begin = useCallback(() => {
     cancel.current?.();
-    cancel.current = startBirthSkyReveal({
-      load: (signal) => loadBirthSkyCover("self:reveal", "self", signal),
-      onReady: (next) => setCover(next),
-      onDiscard: releaseBirthSkyCover,
+    cancel.current = startBirthSkyReveal<BirthSkyRevealSky>({
+      // The overlay chunk loads inside the same budget, so the first frame never waits for code.
+      load: async (signal) => {
+        const [sky] = await Promise.all([
+          loadBirthSkyRevealSky(signal),
+          import("@/components/birth-sky/birth-sky-converge"),
+        ]);
+        return sky;
+      },
+      onReady: (next) => {
+        playing.current = true;
+        setCover(next);
+      },
     });
   }, []);
 
   const close = useCallback(() => {
-    setCover((current) => {
-      releaseBirthSkyCover(current);
-      return null;
-    });
+    if (!playing.current) return;
+    playing.current = false;
+    setCover(null);
+    toast(BIRTH_SKY_REVEAL_HINT);
   }, []);
 
   useEffect(() => () => { cancel.current?.(); }, []);