fix: replace stale RecurrentThreadPtr declaration with std::thread members

The private section still declared the old `thread_` after the method
bodies were updated to use `stop_flag_` and `run_thread_`, leaving the
class in a non-compiling state.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: SHABRAWii

simulate/src/unitree_sdk2_bridge.h

@@ -272,7 +272,8 @@ using WirelessController_t = unitree::robot::go2::publisher::WirelessController;
     std::unique_ptr<LowState_t> lowstate;
 
 private:
-    unitree::common::RecurrentThreadPtr thread_;
+    std::atomic<bool> stop_flag_{false};
+    std::thread run_thread_;
 };
 
 using Go2Bridge = RobotBridge<unitree::robot::go2::subscription::LowCmd, unitree::robot::go2::publisher::LowState>;