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/validation/test_mission.py

@@ -210,7 +210,7 @@ def main():
         target_gps = (target_lat, target_lon, BASE_ALTITUDE)
         print(f"  目標點: ({target_lat:.6f}, {target_lon:.6f})")
 
-        waypoints_per_drone, origin = planner.plan_formation_mission(
+        waypoints_per_drone, origin, _ = planner.plan_formation_mission(
             drone_gps_positions, target_gps, MissionType.M_FORMATION
         )
 
@@ -232,7 +232,7 @@ def main():
         target_gps = (rect_center_lat, rect_center_lon, BASE_ALTITUDE)
         print(f"  矩形中心: ({rect_center_lat:.6f}, {rect_center_lon:.6f})")
 
-        waypoints_per_drone, origin = planner.plan_formation_mission(
+        waypoints_per_drone, origin, _ = planner.plan_formation_mission(
             drone_gps_positions, target_gps, MissionType.GRID_SWEEP,
             params={
                 'rect_corners': rect_corners,