fix: navigate 后立即推一帧，静态页开流不再等重绘

screencast 帧只在重绘时来，页面可能在 startScreencast 之前
就已画好——静态页于是开流后一直无画面，直到下一次重绘。
navigate 后用 CDP captureScreenshot 抓一张当前视口塞进帧
队列，泵在下一轮就把桥帧文件写出来；泵跳过空 session 的
ack（eager 帧没有可确认的 screencast 帧）。失败静默，
screencast 本身仍照常流动。

cloud/playwright_browser.py

@@ -120,6 +120,7 @@ class Browser:
                             wait_until="domcontentloaded")
             self._current_url = self._page.url
             self._start_screencast()
+            self._eager_frame()
         except Exception as exc:  # noqa: BLE001
             raise BrowserError("navigate failed: %s" % exc)
         result = self._info()
@@ -321,9 +322,35 @@ class Browser:
             self._cdp = None
             self._cdp_page = None
 
+    def _eager_frame(self):
+        """Push one current-viewport frame into the sink, best-effort.
+
+        Screencast frames only arrive on repaint, and the page may
+        already have painted before startScreencast ran -- so a
+        static page would otherwise show nothing until its next
+        repaint. Capturing the viewport once right after navigate
+        guarantees the bridge file (and any open stream) has a
+        picture within milliseconds of the page loading.
+        """
+        if self._cdp is None or self._frame_sink is None:
+            return
+        try:
+            result = self._cdp.send(
+                "Page.captureScreenshot",
+                {"format": "jpeg", "quality": 60})
+            data = result.get("data") if isinstance(result, dict) \
+                else None
+            if data:
+                # Empty session id: pump stores it but must not ack
+                # it (there is no screencast frame to acknowledge).
+                self._frame_sink.put(("", data))
+        except Exception:  # noqa: BLE001 - screencast still flows
+            pass
+
     def pump_screencast(self):
         """Main-thread only: store the latest frame and ack every
-        queued frame (no ack = CDP stops sending after the first).
+        real screencast frame (no ack = CDP stops sending after the
+        first). Eager frames carry no session id and skip ack.
         Returns the latest frame data (base64 jpeg) or ''."""
         if self._frame_sink is None:
             return ""
@@ -335,7 +362,7 @@ class Browser:
                 break
             if data:
                 latest = data
-            if self._cdp is not None:
+            if self._cdp is not None and sid:
                 try:
                     self._cdp.send("Page.screencastFrameAck",
                                    {"sessionId": sid})