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/tests/test_playwright.py

@@ -59,12 +59,18 @@ class _FakeCDP:
     def __init__(self):
         self.sent = []
         self.handlers = {}
+        self.fail_capture = False
 
     def on(self, event, handler):
         self.handlers[event] = handler
 
     def send(self, method, params=None):
         self.sent.append((method, params or {}))
+        if method == "Page.captureScreenshot":
+            if self.fail_capture:
+                raise RuntimeError("capture boom")
+            return {"data": "EAGERDATA"}
+        return None
 
 
 class _FakeContext:
@@ -259,6 +265,33 @@ class BrowserTest(unittest.TestCase):
         self.assertIn(("Page.screencastFrameAck", {"sessionId": "s1"}),
                       [(m, p) for m, p in self.page.cdp.sent])
 
+    def test_navigate_pushes_eager_frame_without_ack(self):
+        from playwright_remote import SimpleQueue
+        sink = SimpleQueue()
+        self.browser.set_frame_sink(sink)
+        result = self.browser.navigate("https://example.com")
+        self.assertTrue(result["ok"])
+        self.assertIn(("Page.captureScreenshot",
+                       {"format": "jpeg", "quality": 60}),
+                      [(m, p) for m, p in self.page.cdp.sent])
+        frame = self.browser.pump_screencast()
+        self.assertEqual(frame, "EAGERDATA")
+        acks = [(m, p) for m, p in self.page.cdp.sent
+                if m == "Page.screencastFrameAck"]
+        self.assertEqual(acks, [])
+
+    def test_navigate_survives_failed_eager_capture(self):
+        from playwright_remote import SimpleQueue
+        sink = SimpleQueue()
+        self.browser.set_frame_sink(sink)
+        self.page.cdp.fail_capture = True
+        try:
+            result = self.browser.navigate("https://example.com")
+        finally:
+            self.page.cdp.fail_capture = False
+        self.assertTrue(result["ok"])
+        self.assertEqual(self.browser.pump_screencast(), "")
+
     def test_live_shot_writes_fixed_file_and_status(self):
         self.browser.navigate("https://example.com")
         out = self.browser.live_shot()