rack：fan 命令放行保持时间 sec，风扇断电显示「停」

Co-Authored-By: Claude Opus 5.5 <[email protected]>

rack/backend/prrack/simboard.py

@@ -185,7 +185,10 @@ class SimBoard:
             return {"ok": True, "oldest": 0, "next": len(self.events), "ev": ev}
         if cmd == "fan":
             self.fan_floor = req["duty"]
-            return {"ok": True, "floor": self.fan_floor}
+            rsp = {"ok": True, "floor": self.fan_floor}
+            if "sec" in req:
+                rsp["sec"] = req["sec"]  # 模拟板不倒计时：只把形状回对
+            return rsp
         if req.get("seq") is None:
             return {"ok": False, "err": "no_seq"}
         if cmd == "hub":  # 0.1.14：固件 hubctl.h 的命令形状；不模拟倒计时