架构改进：头部追踪 + 球过时检测 + 球员独占 + 定位球处理

头部追踪：
- 球可见时 he1/he2 追踪球方位，球不可见时左右扫描
- 在技能执行后、commit_motor_targets_pd 前覆盖头部电机目标

球位置过时检测：
- world.py 新增 ball_age property 和阈值常量
- 球消失 ≥3s 时外场球员原地旋转搜索，门将不再基于过期信息拦截

最近球员独占：
- 每个 agent 自行判断是否离球最近（视觉队友位置 + 0.5m 死区防抖）
- 最近球员执行 carry_ball，其他球员走到编号对应的支援站位

定位球基础处理：
- THEIR_KICK 时外场球员退到中圈半径外防守位，面朝球
- OUR_KICK 时最近球员上前踢球，其他人进攻支援位

决策层重构：
- update_current_behavior 外场分支拆分为 playmode_group 判断
- 新增 _outfield_decide / _am_i_nearest_to_ball / support_position / _defensive_set_piece

world/world.py

@@ -1,4 +1,4 @@
-from dataclasses import Field
+from world.commons.field import Field
 import numpy as np
 from world.commons.other_robot import OtherRobot
 from world.commons.field import FIFAField, HLAdultField, Soccer7vs7Field
@@ -38,10 +38,14 @@ class World:
         self.score_left: int = None
         self.score_right: int = None
         self.their_team_name: str = None
-        self.last_server_time: str = None
+        self.last_server_time: float = None
         self._global_cheat_position: np.ndarray = np.zeros(3)
         self.global_position: np.ndarray = np.zeros(3)
         self.ball_pos: np.ndarray = np.zeros(3)
+        self.ball_last_pos: np.ndarray = np.zeros(3)
+        self.ball_last_update_time: float = None
+        self.ball_velocity_2d: np.ndarray = np.zeros(2)
+        self.ball_speed: float = 0.0
         self.is_ball_pos_updated: bool = False
         self.our_team_players: list[OtherRobot] = [OtherRobot() for _ in range(self.MAX_PLAYERS_PER_TEAM)]
         self.their_team_players: list[OtherRobot] = [OtherRobot(is_teammate=False) for _ in
@@ -56,6 +60,19 @@ class World:
             playmode=self.playmode, is_left_team=self.is_left_team
         )
 
+    # ------------------------------------------------------------------
+    # Ball freshness
+    # ------------------------------------------------------------------
+    BALL_FRESH_THRESHOLD: float = 0.6    # < 0.6s: reliable
+    BALL_STALE_THRESHOLD: float = 3.0    # > 3.0s: completely lost
+
+    @property
+    def ball_age(self) -> float:
+        """Seconds since the ball position was last updated by vision."""
+        if self.ball_last_update_time is None or self.server_time is None:
+            return float("inf")
+        return self.server_time - self.ball_last_update_time
+
     def is_fallen(self) -> bool:
         return self.global_position[2] < 0.3
 
@@ -65,4 +82,4 @@ class World:
         elif field_name in ('sim3d_7vs7'):
             return Soccer7vs7Field(world=self)
         else:
-            return FIFAField(world=self)
+            return FIFAField(world=self)