Harden live camera viewer sync rendering

tests/e2e/app.spec.ts

@@ -88,12 +88,54 @@ test("live camera switches into viewer mode when another device already owns ana
   await expect(page.getByTestId("live-camera-viewer-sync-card")).toContainText(
     "均衡模式"
   );
+  await expect(page.getByTestId("live-camera-viewer-sync-card")).toContainText(
+    "已累积"
+  );
   await expect(page.getByTestId("live-camera-viewer-sync-card")).toContainText(
     "猩猩"
   );
   await expect(page.getByTestId("live-camera-score-overall")).toBeVisible();
 });
 
+test("live camera viewer tolerates legacy segments and shows remaining buffer hint", async ({
+  page,
+}) => {
+  const state = await installAppMocks(page, {
+    authenticated: true,
+    liveViewerMode: true,
+  });
+
+  if (state.liveRuntime.runtimeSession?.snapshot) {
+    state.liveRuntime.runtimeSession.snapshot.recentSegments = [
+      {
+        actionType: "forehand",
+        isUnknown: false,
+        startMs: 1200,
+        endMs: 3600,
+        durationMs: 2400,
+        confidenceAvg: 0.82,
+        score: 81,
+        peakScore: 86,
+        frameCount: 18,
+      } as any,
+    ];
+  }
+
+  if (state.mediaSession) {
+    state.mediaSession.durationMs = 4_000;
+    state.mediaSession.playback.previewUrl = undefined;
+  }
+
+  await page.goto("/live-camera");
+  await expect(page.getByText("同步观看模式")).toBeVisible();
+  await expect(
+    page
+      .getByTestId("live-camera-viewer-sync-card")
+      .getByText(/预计还需 6 秒 才会出现首段可观看回放/)
+  ).toBeVisible();
+  await expect(page.getByText("关键帧 0")).toBeVisible();
+});
+
 test("live camera recovers mojibake viewer titles before rendering", async ({
   page,
 }) => {