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/mission_group.py

@@ -35,6 +35,7 @@ class MissionGroup:
         self.planned_waypoints = None     # 規劃結果 dict
         self.executor = None              # MissionExecutor 實例（延遲建立）
         self.center_origin = None         # 規劃原點
+        self.leader_drone_id = None       # LEADER_FOLLOWER 專用：指定的領隊無人機 ID
 
     @property
     def state(self):
@@ -415,6 +416,21 @@ class GroupPanel(QWidget):
                 self._param_widgets[key] = (row_w, inp)
                 self._param_rows.append(row_w)
 
+        # LEADER_FOLLOWER 專用：領隊下拉選單
+        self._leader_row = QWidget()
+        leader_layout = QHBoxLayout(self._leader_row)
+        leader_layout.setContentsMargins(0, 0, 0, 0)
+        leader_layout.setSpacing(3)
+        leader_lbl = QLabel("領隊")
+        leader_lbl.setStyleSheet(LBL)
+        self._leader_combo = QComboBox()
+        self._leader_combo.setStyleSheet(COMBO)
+        self._leader_combo.setFixedWidth(70)
+        self._leader_combo.currentTextChanged.connect(self._on_leader_changed)
+        leader_layout.addWidget(leader_lbl, 1)
+        leader_layout.addWidget(self._leader_combo)
+        param_col.addWidget(self._leader_row)
+
         param_col.addStretch()
 
         # 初始顯示對應的參數
@@ -447,6 +463,27 @@ class GroupPanel(QWidget):
             self.drone_list_label.setText("、".join(sorted_ids))
             self.drone_list_label.setStyleSheet(
                 f"color: {self.group.color}; font-size: 11px; font-weight: bold;")
+        self._refresh_leader_options()
+
+    def _refresh_leader_options(self):
+        """依目前群組成員重新填充領隊下拉選單，保留目前選擇若仍有效"""
+        sorted_ids = sorted(self.group.drone_ids,
+                            key=lambda x: (x.split('_')[0], int(x.split('_')[1])))
+        current = self.group.leader_drone_id
+        self._leader_combo.blockSignals(True)
+        self._leader_combo.clear()
+        self._leader_combo.addItems(sorted_ids)
+        if current and current in sorted_ids:
+            self._leader_combo.setCurrentText(current)
+        elif sorted_ids:
+            self._leader_combo.setCurrentText(sorted_ids[0])
+            self.group.leader_drone_id = sorted_ids[0]
+        else:
+            self.group.leader_drone_id = None
+        self._leader_combo.blockSignals(False)
+
+    def _on_leader_changed(self, drone_id):
+        self.group.leader_drone_id = drone_id if drone_id else None
 
     def update_status(self):
         """更新任務狀態顯示"""
@@ -485,6 +522,7 @@ class GroupPanel(QWidget):
         visible_keys = {d[0] for d in self._param_defs.get(mission_type, [])}
         for key, (row_w, _inp) in self._param_widgets.items():
             row_w.setVisible(key in visible_keys)
+        self._leader_row.setVisible(mission_type == 'LEADER_FOLLOWER')
 
     def get_mission_params(self):
         """讀取當前顯示的參數值，回傳 dict"""