fix: MCP 主循环 stdin 批量行饥饿

select + buffered readline 会丢批量到达的行：一行 readline
可能把多行一起读进用户态 buffer，之后 select 再也看不到
空 fd，buffer 里的行永远不被读。客户端在 initialize 后
习惯把 notification + 下一请求连写，MCP 于是只答 initialize
就卡死（生产 2ff8975 活体验证两跑复现，faulthandler 证实
主线程停在 select）。改用非阻塞 fd + 自有 buffer 的行泵，
每行按到达顺序必达；附带回归测试（批量行全达 + EOF 残行）。

cloud/tests/test_playwright.py

@@ -359,5 +359,63 @@ class MCPTest(unittest.TestCase):
             self.assertIn(name, names)
 
 
+class LineReaderTest(unittest.TestCase):
+    """Pipelined stdin lines must all arrive (no select starvation).
+
+    Regression test for the live failure where the MCP answered
+    initialize and then hung forever: the client wrote
+    notifications/initialized + tools/call back-to-back, one
+    readline() buffered both, and select() never saw the second
+    line again because the fd was drained.
+    """
+
+    def test_pipelined_lines_all_delivered(self):
+        import time
+        from playwright_mcp import _LineReader
+        rfd, wfd = os.pipe()
+        try:
+            reader = _LineReader(rfd)
+            os.write(wfd, b"init\n")
+            lines, drained = reader.poll()
+            self.assertEqual(lines, ["init"])
+            self.assertFalse(drained)
+            # Second batch arrives as one write, like a real client
+            # sending notification + next request back-to-back.
+            os.write(wfd, b"notif\nnavigate\n")
+            # Simulate the drain-loop delay: both lines sit in the
+            # pipe before the next poll.
+            time.sleep(0.2)
+            got = []
+            deadline = time.time() + 2
+            while len(got) < 2 and time.time() < deadline:
+                lines, drained = reader.poll()
+                got.extend(lines)
+                if not lines:
+                    time.sleep(0.02)
+            self.assertEqual(got, ["notif", "navigate"])
+        finally:
+            os.close(wfd)
+            os.close(rfd)
+
+    def test_partial_line_flushed_at_eof(self):
+        from playwright_mcp import _LineReader
+        rfd, wfd = os.pipe()
+        try:
+            reader = _LineReader(rfd)
+            os.write(wfd, b"partial-no-newline")
+            os.close(wfd)
+            lines, drained = reader.poll()
+            self.assertEqual(lines, [])
+            self.assertFalse(drained)
+            lines, drained = reader.poll()
+            self.assertEqual(lines, ["partial-no-newline"])
+            self.assertTrue(drained)
+        finally:
+            try:
+                os.close(rfd)
+            except OSError:
+                pass
+
+
 if __name__ == "__main__":
     unittest.main()