perf: screencast 隔帧改每帧，跟手优先

Page.startScreencast everyNthFrame 2 -> 1，画质 q60、尺寸
1280x800 不动。静态页不产生额外成本（帧只在重绘时来，
且 #187 起桥只写变化的帧字节）；收益全部落在真正需要跟
手的时候：滚动、拖拽、动画期间帧率翻倍。测试断言同步。

cloud/playwright_browser.py

@@ -312,10 +312,16 @@ class Browser:
                     pass
 
             cdp.on("Page.screencastFrame", _on_frame)
+            # everyNthFrame 1: full frame rate during motion.
+            # Static pages cost nothing extra (frames only arrive
+            # on repaint, and the bridge writes only changed
+            # bytes), so dropping the frame-skip buys
+            # responsiveness exactly when it matters -- scrolling,
+            # dragging, animation -- without a static-page tax.
             cdp.send("Page.startScreencast",
                      {"format": "jpeg", "quality": 60,
                       "maxWidth": 1280, "maxHeight": 800,
-                      "everyNthFrame": 2})
+                      "everyNthFrame": 1})
             self._cdp = cdp
             self._cdp_page = self._page
         except Exception:  # noqa: BLE001 - screencast is best-effort