perf: 桥变化才写、stdin 优先、stream 50ms 轮询

1. 桥写放大：旧代码每 ~50ms 轮询都无条件重写 state 和帧
   文件（约 40 次原子 temp+replace/s 走 runsc gofer，
   泵持久返回同一帧、updated_at 每轮都变，字节其实不变）。
   现帧字节不变不写；state 只在 url/title/started/holder/
   streaming 变化时写，或 0.5s 心跳一次（判死阈值 30s，
   绰绰有余）；静态页写量从 ~40次/s 降到 ~2次/s。
2. MCP 主循环 stdin 优先：旧循环先 drain 再读 stdin，
   每次工具调用都先付一轮文件 IO。现 pending 请求先处理，
   空闲轮询 drain，忙时 200ms 保底 drain 一次防饿死。
3. stream 轮询 200ms → 50ms，画面延迟下限随之下降。
附回归测试：无变化轮询不重写、帧变化立即写、holder
变化绕过节流立即写。

cloud/playwright_mcp.py

@@ -430,14 +430,23 @@ def main():
     if _get_bridge(browser) is not None:
         browser.set_frame_sink(_frame_queue())
     reader = _LineReader(sys.stdin.fileno())
+    last_drain = 0.0
     while True:
-        _drain_remote(browser)
         lines, drained = reader.poll()
         for line in lines:
             line = line.strip()
             if not line:
                 continue
             _handle_line(line, browser)
+        # Pending requests go first: a tool call used to pay a
+        # full drain round of file I/O before executing. Drain
+        # on idle iterations as before, plus a 200ms floor while
+        # busy so a request burst can never starve remote input
+        # (takeover/click) or the state heartbeat for long.
+        now = time.time()
+        if not lines or now - last_drain >= 0.2:
+            _drain_remote(browser)
+            last_drain = now
         if drained:
             break
         if not lines: