feat(mission): virtual-leader path following + 固定領隊 + barrier

- mission_planner: Leader-Follower 改用 arc-length 參數化（virtual
  leader），_build_center_path 以同心圓弧連接直線段，銳角退化成 hover
  單點並在該點放 rendezvous barrier，避免暴衝與編隊交叉
- mission_planner: rank 改以輸入順序為準，不再對投影排序，避免浮點噪音
  導致 run-to-run leader 漂移
- mission_group / gui: 新增 leader_drone_id 與「領隊」下拉選單，handle_
  route_confirmed 把指定領隊重排到 rank 0，整個縱隊順序固定如軍隊行進
- mission_executor: rendezvous barrier 釋放邏輯、fallback LOITER、send
  retry；加 barrier_timeout 保護
- command_sender / communication / navigation: 配合 setpoint 送達與
  Ros2 bridge 更新

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

src/GUI/communication.py

@@ -36,6 +36,16 @@ except ImportError as e:
     traceback.print_exc()
     CommandLongClient = None
 
+# 導入 fc_network_apps 的 navigation（PositionTargetGlobalInt / Offboard goto）
+try:
+    from fc_network_apps.navigation import PositionTargetGlobalIntClient
+except ImportError as e:
+    import traceback
+    print(f"⚠️  警告: 無法導入 PositionTargetGlobalIntClient")
+    print(f"   错误: {e}")
+    traceback.print_exc()
+    PositionTargetGlobalIntClient = None
+
 class DroneSignals(QObject):
     update_signal = pyqtSignal(str, str, object)  # (msg_type, drone_id, data)
 
@@ -475,6 +485,13 @@ class DroneMonitor(Node):
         self.client_counter = 0  # 用於生成唯一的 client 節點名稱
         self.executor = None  # 將在 gui.py 中設置，用於添加新的 clients
         # ================================================================================
+
+        # ================================================================================
+        # PositionTargetGlobalIntClient 字典（per-drone，用於 Offboard goto）
+        # ================================================================================
+        self.position_target_clients = {}  # {drone_id: PositionTargetGlobalIntClient}
+        self.pos_client_counter = 0
+        # ================================================================================
         
         # 主题检测定时器
         self.create_timer(1.0, self.scan_topics)
@@ -531,7 +548,27 @@ class DroneMonitor(Node):
                     print(f"⚠️  無法為 {drone_id} 創建 CommandLongClient: {e}")
                     return None
             return self.command_long_clients[drone_id]
-    
+
+    def get_or_create_position_client(self, drone_id):
+        """為每個 drone 獲取或創建獨立的 PositionTargetGlobalIntClient。"""
+        if PositionTargetGlobalIntClient is None:
+            return None
+        with self.client_lock:
+            if drone_id not in self.position_target_clients:
+                try:
+                    self.pos_client_counter += 1
+                    unique_name = f"pos_target_client_{drone_id}_{self.pos_client_counter}"
+                    client = PositionTargetGlobalIntClient(node_name=unique_name)
+                    self.position_target_clients[drone_id] = client
+                    print(f"   ✓ 為 {drone_id} 創建新的 PositionTargetGlobalIntClient (node={unique_name})")
+                    if self.executor:
+                        self.executor.add_node(client)
+                        print(f"   ✓ 將 {drone_id} 的 position client 添加到主執行器")
+                except Exception as e:
+                    print(f"⚠️  無法為 {drone_id} 創建 PositionTargetGlobalIntClient: {e}")
+                    return None
+            return self.position_target_clients[drone_id]
+
     def scan_topics(self):
         topics = self.get_topic_names_and_types()
         drone_pattern = re.compile(r'/fc_network/vehicle/(sys\d+)/(\w+)')