feat(web): D4 会话未读徽标 — 打开即已读 + 消费 session_read

- SessionMeta.unreadCount（可选，兼容旧后端）+ api/sessions.markSessionRead
- lib/sessionRead.ts：会话历史落地后再推游标（best-effort，失败静默，下次 GET /sessions 收敛）+ 帧归属判定（ws.ts 因此留在 500 行内）
- store.clearUnread 无变化时返回同一 state，避免多余重渲染；Nav 徽标沿用 Miuix token（mu-radius-pill/mu-err/mu-rise-in）
- 逻辑测试 35 → 54 项，含变异校验（改成 clearUnread(activeSessionId) 即 FAIL）

frontend/web/src/lib/ws.ts

@@ -14,6 +14,7 @@ import {
   toRecord,
 } from "./wsParse";
 import { MODES, type AwakenState, type ChatMessage, type Mode, type WsUpEvent } from "./types";
+import { sessionReadTarget } from "./sessionRead";
 import { streamFrameApplies } from "./streamScope";
 
 /**
@@ -321,6 +322,15 @@ export function dispatchFrame(ev: { type: string; sessionId?: string; data: unkn
       s.setSessionBusy(busy, pendingCount);
       break;
     }
+    case "session_read": {
+      // 协同 B3/D4 — one end marked the conversation read, so the unread badge goes
+      // out on every end. Only the addressed session is touched (a frame naming
+      // another conversation must not clear the dot of the one on screen), and an
+      // unattributable frame is dropped rather than guessed.
+      const id = sessionReadTarget(ev.sessionId, r);
+      if (id) s.clearUnread(id);
+      break;
+    }
     default: {
       // Unknown future event: keep it in the context log so no frame is lost.
       const text = typeof ev.data === "string" ? ev.data : JSON.stringify(ev.data);