feat(web): 前端 P0 收尾（多路忙态收口 / 握手 history 落 store / 清理幽灵键）

frontend/web/src/lib/ws/handlers/message.ts

@@ -1,6 +1,11 @@
 import { MODES, type ChatMessage, type Mode } from "../../types";
-import { parseAdminChannelMessage, parsePersistedMessage, toRecord } from "../../wsParse";
-import type { FrameEvent, FramePayload, StoreState } from "./shared";
+import {
+  parseAdminChannelMessage,
+  parseHistoryMessages,
+  parsePersistedMessage,
+  toRecord,
+} from "../../wsParse";
+import { frameSessionId, type FrameEvent, type FramePayload, type StoreState } from "./shared";
 
 /** The fields of a `message_edited` payload the chat needs: the edited message's
  *  id and its new content. `sessionId` is carried inside the broadcast DTO (and on
@@ -40,6 +45,18 @@ function parseMessageEdited(data: unknown): MessageEdited | null {
  *  未命中返回 false 交给下一组。 */
 export function handleMessageEvent(ev: FrameEvent, r: FramePayload, s: StoreState): boolean {
   switch (ev.type) {
+    case "history": {
+      // 握手回放（契约 §3.2，仅 owner/admin，只回放主会话最近 50 条）：此前这帧落到
+      // 未知帧兜底被丢掉 ⇒ WS 断线窗口内落库的消息永久缺失（要等切会话/刷新才补）。
+      // 并入时按 id 去重、只按精确键（clientMsgId / turnId）认领本端气泡（见
+      // sessionCache.mergeHistory）—— 整批回放绝不用内容/时间启发式，以免把正在
+      // 流式的气泡错配到某条旧行上。帧壳 sessionId（"web"）就是回放所属会话。
+      const msgs = parseHistoryMessages(r?.messages);
+      if (msgs.length) {
+        s.mergeHistoryPage(frameSessionId(ev, r) ?? msgs[0].sessionId ?? "", msgs);
+      }
+      return true;
+    }
     case "message_persisted": {
       // design/16 §4.2 — a user or assistant message was persisted and broadcast
       // with its backend id. 裁决 16.4.5: the frame is written into the bucket its