feat(flow): 回调信封升级——事件携带操作人身份（operatorUserId/operatorName）

在飞工作合入：FlowEvent 增操作人字段（可空），引擎/推进监听/回调派发全程透传，
对齐 SDK FlowCallbackEvent 的 taskId/operator 字段（规范 §6.4）。

src/main/java/com/yunhai/flowcenter/engine/FlowEvent.java

@@ -59,14 +59,24 @@ public class FlowEvent {
     private final Long instanceId;
     /** 任务事件带 taskId（SUBMITTED 等办件级事件为 null） */
     private final Long taskId;
+    /** 操作人 id（谁导致本事件；回调信封提升为 operatorUserId）。可空：语义见《子应用接入流程中心规范.md》§6.4 */
+    private final Integer operatorId;
+    /** 操作人姓名快照。可空 */
+    private final String operatorName;
     /** 事件明细（result/opinion/rejectNode/task.created 的 n/m 等），可为空 */
     private final Map<String, Object> data;
 
     public static FlowEvent of(Type type, Long instanceId) {
-        return new FlowEvent(type, instanceId, null, new HashMap<>());
+        return new FlowEvent(type, instanceId, null, null, null, new HashMap<>());
     }
 
     public static FlowEvent of(Type type, Long instanceId, Long taskId, Map<String, Object> data) {
-        return new FlowEvent(type, instanceId, taskId, data == null ? new HashMap<>() : data);
+        return new FlowEvent(type, instanceId, taskId, null, null, data == null ? new HashMap<>() : data);
+    }
+
+    public static FlowEvent of(Type type, Long instanceId, Long taskId,
+                               Integer operatorId, String operatorName, Map<String, Object> data) {
+        return new FlowEvent(type, instanceId, taskId, operatorId, operatorName,
+                data == null ? new HashMap<>() : data);
     }
 }