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_remote.py

@@ -14,12 +14,12 @@ is four regular files in the worktree, all written atomically
 (temp file + os.replace) except the append-only input log:
 
 - ``.browser-remote-state.json``: {url, title, started, holder,
-  streaming, updated_at} -- rewritten by the MCP drain loop on
-  every iteration, so its mtime is the session-liveness signal.
-  The server treats a state file older than STATE_STALE_SECONDS
-  as a dead session (503).
+  streaming, updated_at} -- rewritten when a meaningful field
+  changes or at the heartbeat interval, so its updated_at is the
+  session-liveness signal. The server treats a state file older
+  than STATE_STALE_SECONDS as a dead session (503).
 - ``.browser-remote-frame.jpg``: latest screencast frame, base64
-  text, rewritten only when a new frame arrives.
+  text, rewritten only when its bytes change.
 - ``.browser-remote-input.jsonl``: the server appends one JSON
   object per line. The MCP tracks a byte offset and executes
   complete lines only (a trailing partial line waits for the
@@ -34,10 +34,10 @@ Threading contract (unchanged from the socket version, measured
 2026-10-30: Playwright sync calls from any thread but the
 creating one raise "Cannot switch to a different thread"):
 only the MCP main thread touches the browser. It calls
-RemoteBridge.poll() on every select() iteration (~50ms), so
-control/input/frames flow even while the agent is idle, and CDP
-frame acks keep flowing (no ack = CDP stops after the first
-frame).
+RemoteBridge.poll() on every idle main-loop iteration (~50ms),
+so control/input/frames flow even while the agent is idle, and
+CDP frame acks keep flowing (no ack = CDP stops after the
+first frame).
 
 Control model (unchanged): holder "agent" by default; holder
 "user" while the console user has taken over, during which
@@ -55,6 +55,10 @@ FRAME_NAME = ".browser-remote-frame.jpg"
 INPUT_NAME = ".browser-remote-input.jsonl"
 CONTROL_NAME = ".browser-remote-control.json"
 STATE_STALE_SECONDS = 30
+# Heartbeat interval for state writes. The server treats state
+# older than STATE_STALE_SECONDS as dead, so 0.5s is plenty;
+# writing every ~50ms iteration was pure gofer round-trips.
+STATE_HEARTBEAT_SECONDS = 0.5
 
 
 class SimpleQueue:
@@ -114,6 +118,9 @@ class RemoteBridge:
         self._input_offset = 0
         self._control_seq = None
         self._frame_written = False
+        self._last_frame = None
+        self._last_state_key = None
+        self._last_state_write = 0.0
 
     def _path(self, name):
         return os.path.join(self.workdir, name)
@@ -180,7 +187,18 @@ class RemoteBridge:
                 print("playwright_remote: input error: %s" % exc)
 
     def poll(self, browser=None):
-        """One drain-loop iteration: control, inputs, frame, state."""
+        """One drain-loop iteration: control, inputs, frame, state.
+
+        Writes are change-gated. The old code rewrote both files
+        on every ~50ms iteration: ~20 atomic temp+replace
+        round-trips per second through the runsc gofer mount for
+        identical bytes (pump returns the latest frame
+        persistently, and updated_at changes every call). Now the
+        frame file is rewritten only when its bytes change, and
+        the state file only when a meaningful field changes or
+        the heartbeat interval elapsed -- a static page costs
+        ~2 writes per second instead of ~40.
+        """
         target = browser if browser is not None else self.browser
         self._poll_control()
         self._poll_inputs()
@@ -188,20 +206,30 @@ class RemoteBridge:
             frame = target.pump_screencast()
         except Exception:  # noqa: BLE001 - screencast best-effort
             frame = ""
-        if frame:
+        if frame and frame != self._last_frame:
             try:
                 _atomic_write(self._path(FRAME_NAME),
                               frame.encode("utf-8"))
+                self._last_frame = frame
                 self._frame_written = True
             except OSError:
                 pass
-        try:
-            _atomic_write(
-                self._path(STATE_NAME),
-                json.dumps(self.state_dict(),
-                           ensure_ascii=False).encode("utf-8"))
-        except OSError:
-            pass
+        state = self.state_dict()
+        key = (state.get("url"), state.get("title"),
+               state.get("started"), state.get("holder"),
+               state.get("streaming"))
+        now = state["updated_at"]
+        if key != self._last_state_key or \
+                now - self._last_state_write >= STATE_HEARTBEAT_SECONDS:
+            try:
+                _atomic_write(
+                    self._path(STATE_NAME),
+                    json.dumps(state,
+                               ensure_ascii=False).encode("utf-8"))
+                self._last_state_key = key
+                self._last_state_write = now
+            except OSError:
+                pass
 
 
 def bridge_paths(workdir):