feat(enemy): Wander 巡逻改为当前段内随机 + 按身体宽度内缩两端

修复地面单位 Wander 巡逻"走一小段就停"：
- 随机点从全图(GetRandomPointOnGraph)改为限定在 agent 当前所站的 NavSegment 上,
  不再跨越跳/落/拐角 NavLink,避免路径跨沟被崖边夹停或卡在过不去的 link。
- 段内随机 t 按"身体前缘安全余量(碰撞体半宽 + 悬崖检测前向偏移)"从两端各内缩,
  避免随机点落在角色宽度站不住的崖沿(WouldFallAhead 夹停、IsFollowingAPath 不复位→卡死)。
- NavAgent.SetRandomDestinationOnCurrentSegment(endMargin):段内采样+内缩(段过短退化到中点)
- EnemyMovement.EdgeSafeMargin:身体前缘安全余量
- IPathAgent.WalkToRandomOnSegment():EnemyNavAgent 传入 margin;飞行单位复用局部偏移;Null 返回 false

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Assets/PathBerserker2d/Scripts/PathBerserker2d/NavAgent/NavAgent.cs

@@ -594,6 +594,32 @@ namespace PathBerserker2d
             return PathTo(goal);
         }
 
+        // [项目新增] 段内随机游走：目标点限制在 agent 当前所站的这一条 NavSegment(cluster) 上，
+        // 不会跨越任何 NavLink(跳/落/拐角)，避免地面单位游走时被崖边夹停或卡在无法穿越的 link 处。
+        // 需要 agent 当前已映射到某条段上(currentMappedPosition 有效)，否则返回 false。
+        // 注意：一条 cluster 是一条直线段；被拐角/斜坡拆成多段的平台只会在当前这一段内游走。
+        /// <summary>
+        /// Start pathfinding to a random position ON THE SAME segment the agent currently stands on.
+        /// Never crosses a NavLink, so a ground unit stays on its current platform segment.
+        /// <paramref name="endMargin"/> insets the random range from BOTH segment ends (world units),
+        /// so the goal is never so close to the edge that a bodied agent can't stand there
+        /// (pass the agent's half-width + ledge-check margin). If the segment is shorter than
+        /// 2*endMargin the goal degenerates to the segment midpoint.
+        /// Returns false if the agent is not currently mapped to a segment.
+        /// </summary>
+        public bool SetRandomDestinationOnCurrentSegment(float endMargin = 0f)
+        {
+            if (currentMappedPosition.IsInvalid())
+                return false;
+
+            var cluster = currentMappedPosition.cluster;
+            float len = cluster.Length;
+            float m = Mathf.Clamp(endMargin, 0f, len * 0.5f); // 段太短 → 退化到中点
+            float t = UnityEngine.Random.Range(m, len - m);
+            Vector2 goal = cluster.owner.LocalToWorld.MultiplyPoint3x4(cluster.GetPositionAlongSegment(t));
+            return PathTo(goal);
+        }
+
 
         /// <summary>
         /// If you implement link traversal yourself, call this to complete a link traversal.