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/sessionRead.ts

@@ -0,0 +1,45 @@
+import { useAppStore } from "../store/useAppStore";
+import { markSessionRead } from "./api";
+
+/**
+ * Session read cursor (Android 协同 B3/D4) — the consumer side of the per-session
+ * unread count. Two write paths, both landing on the same single cursor:
+ *
+ *   - opening a session (this end)      → POST /sessions/{id}/read
+ *   - the `session_read` frame (other ends) → clears the badge locally only
+ *
+ * The store's `clearUnread` zeroes the badge; the network cannot fail the badge,
+ * because `GET /sessions` is the eventual truth (a failed POST simply means the
+ * dot comes back on the next list load).
+ */
+
+/** Which session does a `session_read` frame address? The payload's own
+ *  `sessionId` wins when present (a frame may scope itself in the body, like the
+ *  streaming frames), else the envelope's. "" means unattributable: the caller
+ *  must then leave every badge alone rather than guess, since clearing the wrong
+ *  session's badge would hide messages the user has not seen. */
+export function sessionReadTarget(
+  envelopeSessionId: string | undefined,
+  r: Record<string, unknown> | null
+): string {
+  const payload = typeof r?.sessionId === "string" ? r.sessionId : "";
+  return payload || envelopeSessionId || "";
+}
+
+/** Mark a session read because the user just opened it (called once its first
+ *  page of history has landed, so the cursor never passes messages that were
+ *  never rendered). Zeroes the badge immediately — the list must not show a stale
+ *  dot for the conversation on screen — and pushes the cursor so every other end
+ *  converges. A session this end already reads as read is skipped entirely: the
+ *  WS frame (or this very call) may have cleared it first, and switching back and
+ *  forth must not fire a request per render. */
+export function markSessionOpenedRead(sessionId: string): void {
+  if (!sessionId) return;
+  const { sessions, clearUnread } = useAppStore.getState();
+  if (!(sessions[sessionId]?.unreadCount ?? 0)) return;
+  clearUnread(sessionId);
+  void markSessionRead(sessionId).catch(() => {
+    // Best effort: the server cursor stays where it was until the next attempt,
+    // and the next GET /sessions reports it as truth.
+  });
+}