feat(ai): BrainBuilder.DeclareState——重复声明状态即报错，防模块间同名静默覆盖

State() 仍保持宽松(有则取),用于给已声明的态挂边;DeclareState() 只用于
首次声明行为,重复声明同名状态会抛异常。AiStateFragments 的四个建态原语
(Locomotion/Ability/AbilityOnce/Terminal)改用 DeclareState。同时给
IEngagementModule/IDeathModule/IUnawareModule 补充不变量说明。

Assets/_Game/Scripts/Enemies/AIBrain/Modules/AiStateFragments.cs

@@ -15,7 +15,7 @@ namespace BaseGames.Enemies
 
         /// <summary>由 EnemyLocomotion 驱动的态：进入 / 每帧声明移动意图，离开时停。</summary>
         public static BrainBuilder.StateBuilder Locomotion(BrainBuilder b, string state, LocomotionMode mode)
-            => b.State(state)
+            => b.DeclareState(state)
                 .OnEnter(x => ApplyLocomotion(x, mode))
                 .Tick  (x => ApplyLocomotion(x, mode))
                 .OnExit(x => x.Locomotion.Stop());
@@ -27,7 +27,7 @@ namespace BaseGames.Enemies
         public static BrainBuilder.StateBuilder Ability(BrainBuilder b, string state, string abilityId)
         {
             RequireAbilityId(state, abilityId);
-            return b.State(state)
+            return b.DeclareState(state)
                 .OnEnter(x => EnsureAbility(x, abilityId))
                 .Tick  (x => EnsureAbility(x, abilityId))
                 .OnExit(x => x.Combat.InterruptAbilities());
@@ -40,7 +40,7 @@ namespace BaseGames.Enemies
         public static BrainBuilder.StateBuilder AbilityOnce(BrainBuilder b, string state, string abilityId)
         {
             RequireAbilityId(state, abilityId);
-            return b.State(state).OnEnter(x => EnsureAbility(x, abilityId));
+            return b.DeclareState(state).OnEnter(x => EnsureAbility(x, abilityId));
         }
 
         /// <summary>
@@ -48,7 +48,7 @@ namespace BaseGames.Enemies
         /// 用于"死亡演出交给物理状态机（EnemyBase.PerformDeath）"的敌人。
         /// </summary>
         public static BrainBuilder.StateBuilder Terminal(BrainBuilder b, string state)
-            => b.State(state);
+            => b.DeclareState(state);
 
         static void ApplyLocomotion(IAiContext x, LocomotionMode mode)
         {