feat(web,ws): 会话级消息缓存 + 图桶 + 忙态按会话过滤（裁决 16.4.5/16.4.2 + 修 16.4.1 的帧缺口）

- 16.4.5：非激活会话的帧**不再丢弃**，按自己的 sessionId 落进该会话桶（新增 store/sessionCache.ts 纯函数 +
  useAppStore.sessionMessages/flowGraphs）；切会话改为「存回/取出」而非清空；流式增量 O(1) 定位桶、
  只延长已存在的助手气泡（不凭一帧 delta 凭空造消息）
- 16.4.2：图按会话入桶（上限 12 个），Backthink 页取 sess-backthink 的图而不再被用户会话图顶替；
  空态区分「实时通道未连接」与「尚未产生链路图」；wire_runtime 的图源白名单注释按裁决口径重写（零行为改动）
- 修 16.4.1 的缺口：handlers/session.ts 补 `session_idle` / `session_interrupted`（此前落到未知帧兜底 ⇒
  sessionBusy 可能卡 true ⇒ 预约永不落地）；忙态**置忙与置闲两侧**统一按 frameIsActive 过滤（只过滤置闲
  会让后台会话的忙帧点亮徽标而它自己的 idle 被丢 ⇒ 永久卡忙）
- 清理：删除 lib/streamScope.ts（口径已被 16.4.5 推翻）与只剩测试在用的 messageEventApplies
- 复核：tsc 0 错 + vite build 成功；messageSync 69 / session 19 项逻辑测试；4 条变异反证

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

@@ -1,12 +1,12 @@
-import { messageEventApplies } from "../../../store/useAppStore";
 import { MODES, type ChatMessage, type Mode } from "../../types";
 import { parseAdminChannelMessage, parsePersistedMessage, toRecord } from "../../wsParse";
 import type { FrameEvent, FramePayload, 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
- *  the frame envelope) so the frame can be filtered; `mode`/`mood` are optional —
- *  an edit never changes them, but the DTO carries them for completeness. */
+ *  the frame envelope) so the store can confine the patch to that session's bucket;
+ *  `mode`/`mood` are optional — an edit never changes them, but the DTO carries them
+ *  for completeness. */
 interface MessageEdited {
   id: string;
   content: string;
@@ -42,39 +42,36 @@ export function handleMessageEvent(ev: FrameEvent, r: FramePayload, s: StoreStat
   switch (ev.type) {
     case "message_persisted": {
       // design/16 §4.2 — a user or assistant message was persisted and broadcast
-      // with its backend id. Only surface messages for the active session; the
-      // store dedups by id and adopts the local optimistic bubble (uid → backend
-      // id), so both the originating and remote browsers converge on one message.
+      // with its backend id. 裁决 16.4.5: the frame is written into the bucket its
+      // own `sessionId` names (the store routes it), so a background conversation
+      // keeps accumulating instead of being dropped. Within that bucket the store
+      // dedups by id and adopts the local optimistic bubble (uid → backend id), so
+      // both the originating and remote browsers converge on one message.
       const msg = parsePersistedMessage(r?.message);
-      if (msg && msg.sessionId === s.activeSessionId) {
-        s.appendOrUpdateMessage(msg);
-      }
+      if (msg) s.appendOrUpdateMessage(msg);
       return true;
     }
     case "message_edited": {
       // design/16 §4.2 (D3 edit path) — the stored message's content was changed
       // and broadcast with the updated DTO. The backend row stays the single source
-      // of truth; this only re-renders the bubble, and only in the session it
-      // belongs to (same filter as message_persisted). Patch by id — an id we do not
-      // hold is another session's (or an already-gone) message, never a new row.
+      // of truth; this only re-renders the bubble of the session the frame names
+      // (the store confines the patch to that session's bucket). Patch by id — an
+      // id we do not hold is another session's (or an already-gone) message, never
+      // a new row.
       const edit = parseMessageEdited(ev.data);
       if (!edit) return true;
-      if (messageEventApplies(edit.sessionId ?? ev.sessionId, s.activeSessionId)) {
-        const patch: Partial<ChatMessage> = { content: edit.content };
-        if (edit.mode) patch.mode = edit.mode;
-        if (edit.mood) patch.mood = edit.mood;
-        s.updateMessageById(edit.id, patch);
-      }
+      const patch: Partial<ChatMessage> = { content: edit.content };
+      if (edit.mode) patch.mode = edit.mode;
+      if (edit.mood) patch.mood = edit.mood;
+      s.updateMessageById(edit.id, patch, edit.sessionId ?? ev.sessionId);
       return true;
     }
     case "message_deleted": {
       // design/16 §4.2 (D3 delete path) — the message was removed server-side, so
-      // drop it by id instead of waiting for a manual re-read. The id is globally
-      // unique; sessionId is advisory and, when present, must be the active one.
+      // drop it by id instead of waiting for a manual re-read. The frame's session
+      // scopes the removal; without one the store searches by id (globally unique).
       const id = r && typeof r.id === "string" ? r.id : "";
-      if (id && messageEventApplies(ev.sessionId, s.activeSessionId)) {
-        s.removeMessageById(id);
-      }
+      if (id) s.removeMessageById(id, ev.sessionId);
       return true;
     }
     case "channel_persisted": {