MutationApplicationContext
data class MutationApplicationContext(val battle: BattleState, val field: FieldState, val units: Map<String, UnitState>, val sides: Map<String, SideState> = emptyMap(), val selfId: String? = null, val targetId: String? = null, val sourceId: String? = null, val side: SideState? = null, val foeSide: SideState? = null)
mutation 应用阶段的上下文。
设计意图：
为 MutationApplier 提供完整的状态快照与 selector 解析信息。
把“状态应用”从 EventContext 解耦，避免 runtime 读上下文和写状态强绑定。