fix: 浏览器远控传输层改文件桥（Unix socket 在 runsc 下宿主不可见）

根因（2026-10-30 活体实证）：#181 的 .browser-remote.sock 假设
宿主能连容器内 socket，但生产 Docker 任务走 runsc/gVisor，
其 gofer 挂载下容器内建的 socket 只在容器内可见，宿主目录
里只有普通文件（browser-live.png 一直能透传）。后果：server
代理对真实任务一律 503，会话根本没活过。

修法：传输层改四个普通文件（worktree 内，原子写 + 追加日志）：
- .browser-remote-state.json：MCP 每轮重写，mtime 即存活信号，
  server 判 30 秒无心跳为会话死亡（503）；
- .browser-remote-frame.jpg：最新 screencast 帧（base64 文本）；
- .browser-remote-input.jsonl：server 追加，MCP 按偏移量消费
  完整行，执行时再判 holder，半行等下一轮；
- .browser-remote-control.json：{holder, seq}，MCP 首轮即应用，
  接管可先于浏览器启动。
- holder 竞态：control 写下后 state 文件约 50ms 才刷新，server
  的 input/stream 改判 effective_holder（控制文件 mtime 新者胜），
  否则接管后立刻点会误 409。
- 线程契约与控制权模型不变；MCP 主循环仍是唯一触页线程。
- 播种刷新（#181）让老租户自动拿到新版脚本。

测试：socket 测试改文件桥测试（桥轮询/偏移量/半行/陈旧态/
effective_holder + server 代理全链），全量 1357 项通过（skipped 4）。

cloud/playwright_mcp.py

@@ -230,7 +230,7 @@ def _err(code, message):
 
 
 _BROWSER = None
-_REMOTE = None
+_BRIDGE = None
 
 
 def _get_browser(browser=None):
@@ -242,23 +242,23 @@ def _get_browser(browser=None):
     return _BROWSER
 
 
-def _get_remote(browser=None):
-    """Remote control state, lazily created. Keyed by browser
+def _get_bridge(browser=None):
+    """File-bridge remote control, lazily created. Keyed by browser
     identity so tests passing different fake browsers don't share
     one holder flag."""
-    global _REMOTE
+    global _BRIDGE
     target = _get_browser(browser)
-    if _REMOTE is not None and _REMOTE.browser is target:
-        return _REMOTE
+    if _BRIDGE is not None and _BRIDGE.browser is target:
+        return _BRIDGE
     try:
-        from playwright_remote import RemoteState
+        from playwright_remote import RemoteBridge
     except ImportError:
         try:
-            from cloud.playwright_remote import RemoteState
+            from cloud.playwright_remote import RemoteBridge
         except ImportError:
             return None
-    _REMOTE = RemoteState(target)
-    return _REMOTE
+    _BRIDGE = RemoteBridge(target, getattr(target, "cwd", None))
+    return _BRIDGE
 
 
 def handle_request(request, browser=None):
@@ -299,7 +299,7 @@ def handle_request(request, browser=None):
         name = params.get("name")
         args = params.get("arguments") or {}
         target = _get_browser(browser)
-        remote = _get_remote(browser)
+        remote = _get_bridge(browser)
         if remote is not None and remote.get_holder() == "user":
             return {"jsonrpc": "2.0", "id": req_id,
                     "result": {"content": [{"type": "text",
@@ -359,61 +359,28 @@ def handle_request(request, browser=None):
 
 
 def _drain_remote(browser):
-    """Main-thread only: run queued user inputs and pump screencast
-    frames (acks). Called on every loop iteration so remote input
+    """Main-thread only: poll the file bridge (control, inputs,
+    frame, state). Called on every loop iteration so remote input
     lands even while no JSON-RPC request is in flight."""
-    remote = _get_remote(browser)
-    if remote is None:
+    bridge = _get_bridge(browser)
+    if bridge is None:
         return
     try:
-        from playwright_remote import execute_input
-    except ImportError:
-        try:
-            from cloud.playwright_remote import execute_input
-        except ImportError:
-            return
-    while True:
-        try:
-            item = remote.inputs.get_nowait()
-        except Exception:  # noqa: BLE001 - queue empty
-            break
-        if remote.get_holder() != "user":
-            continue
-        try:
-            execute_input(browser, item)
-        except Exception as exc:  # noqa: BLE001 - input errors are data
-            print("playwright_mcp: remote input error: %s" % exc,
-                  file=sys.stderr)
-    try:
-        frame = browser.pump_screencast()
-    except Exception:  # noqa: BLE001 - screencast best-effort
-        frame = ""
-    if frame:
-        remote.set_frame(frame)
+        bridge.poll(browser)
+    except Exception as exc:  # noqa: BLE001 - bridge best-effort
+        print("playwright_mcp: bridge poll error: %s" % exc,
+              file=sys.stderr)
 
 
 def main():
     import select
     browser = _get_browser()
-    remote = _get_remote(browser)
-    if remote is not None:
-        try:
-            from playwright_remote import start_remote_server
-        except ImportError:
-            try:
-                from cloud.playwright_remote import start_remote_server
-            except ImportError:
-                start_remote_server = None
-        if start_remote_server is not None:
-            socket_path = os.environ.get(
-                "PLAYWRIGHT_REMOTE_SOCKET",
-                os.path.join(browser.cwd, ".browser-remote.sock"))
-            try:
-                browser.set_frame_sink(_frame_queue())
-                start_remote_server(remote, socket_path)
-            except Exception as exc:  # noqa: BLE001 - remote best-effort
-                print("playwright_mcp: remote server failed: %s" % exc,
-                      file=sys.stderr)
+    # Frame sink must be set before any navigate so pump_screencast
+    # has frames for the bridge to publish. No bridge (missing
+    # module) means no sink either, so frames are dropped instead
+    # of accumulating unboundedly in a queue nobody drains.
+    if _get_bridge(browser) is not None:
+        browser.set_frame_sink(_frame_queue())
     buffer = ""
     while True:
         _drain_remote(browser)