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/verify_waypoints.py

@@ -365,7 +365,7 @@ def visualize_standalone():
     fig.suptitle('Mission Planner Verification (standalone)', fontsize=14, fontweight='bold')
 
     # M-Formation
-    wps_m, origin_m = planner.plan_formation_mission(drones, target, MissionType.M_FORMATION)
+    wps_m, origin_m, _ = planner.plan_formation_mission(drones, target, MissionType.M_FORMATION)
     conv_m = CoordinateConverter(origin_m[0], origin_m[1], 0)
     data_m = {
         'drone_ids': [f's0_{i + 1}' for i in range(len(drones))],
@@ -380,7 +380,7 @@ def visualize_standalone():
     # Grid Sweep 5m
     target_gs = (sum(c[0] for c in rect_corners) / 4,
                  sum(c[1] for c in rect_corners) / 4, 10.0)
-    wps_g, origin_g = planner.plan_formation_mission(
+    wps_g, origin_g, _ = planner.plan_formation_mission(
         drones, target_gs, MissionType.GRID_SWEEP,
         params={'rect_corners': rect_corners, 'line_spacing': 5.0, 'altitude': 10.0}
     )
@@ -403,7 +403,7 @@ def visualize_standalone():
         (24.12410, 120.56800),
         (24.12420, 120.56790),
     ]
-    wps_lf, origin_lf = planner.plan_formation_mission(
+    wps_lf, origin_lf, _ = planner.plan_formation_mission(
         drones, target, MissionType.LEADER_FOLLOWER,
         params={'route_waypoints': route, 'lateral_offset': 3.0,
                 'longitudinal_spacing': 5.0, 'altitude': 10.0}