fix(atsc_app): 暂停FSSI刷新期间的IQ采集

FSSI Refresh开启后，网页提前改写全局状态，使频谱隐藏函数误判状态
未变化而直接返回；同时i.MX仍持续组装IQ窗口并执行FFT和眼图分析。

由频谱页状态函数统一完成前端切换。后端在A/B轮询期间丢弃IQ包，
清除待处理窗口和旧测量结果；关闭FSSI Refresh后，从pkt_idx=0的
新窗口恢复采集，避免混入A/B切换期间或内核积压的旧数据。

验证：node --check src/web/www/js/dashboard.js，通过。
验证：加载Yocto环境后执行make -C src/atsc_app -j2，编译通过。
未部署到192.168.1.143，未执行板端FSSI开关和恢复验证。

src/atsc_app/task/iq_rx.c

@@ -161,11 +161,22 @@ void iq_spec_set_enable(int enable)
 
     iq_spec_enable = enable;
     if (!enable) {
-        /* This switch controls spectrum-page files only.  Status-page SNR and
-         * shoulders must continue to be measured in the background. */
+        /* FSSI A/B switching mixes two feedback paths, so no IQ window collected
+         * in this mode is valid as a stable single-path spectrum or eye. */
+        pthread_mutex_lock(&iq_mtx);
+        iq_ready = -1;
+        pthread_mutex_unlock(&iq_mtx);
+        pthread_mutex_lock(&vsb_mtx);
+        vsb_pending = 0;
+        pthread_mutex_unlock(&vsb_mtx);
+        pthread_mutex_lock(&imd_mtx);
+        imd_stamp = 0;
+        pthread_mutex_unlock(&imd_mtx);
+        iq_vsb_clear_results();
         unlink(IQ_JSON);
         unlink(IQ_JSON_TMP);
         unlink("/tmp/iq_eye.json");
+        unlink("/tmp/iq_raw.bin");
     }
     printf("[iq_rx] spectrum %s\n", enable ? "enabled" : "disabled");
 }
@@ -184,9 +195,10 @@ static void *iq_vsb_thr(void *arg)
         while (!vsb_pending) pthread_cond_wait(&vsb_cond, &vsb_mtx);
         pthread_mutex_unlock(&vsb_mtx);
 
-        iq_vsb_analyse(vsb_win, vsb_wid);
-
-        if (!iq_spec_get_enable()) {
+        if (iq_spec_get_enable()) {
+            iq_vsb_analyse(vsb_win, vsb_wid);
+        } else {
+            iq_vsb_clear_results();
             unlink("/tmp/iq_eye.json");
         }
 
@@ -245,6 +257,13 @@ static void *iq_spec_thr(void *arg)
         buf = iq_ready; npkt = iq_ready_pkts; run = iq_ready_run; wid = iq_ready_wid;
         pthread_mutex_unlock(&iq_mtx);
 
+        if (!iq_spec_get_enable()) {
+            pthread_mutex_lock(&iq_mtx);
+            iq_ready = -1;
+            pthread_mutex_unlock(&iq_mtx);
+            continue;
+        }
+
         for (i = 0; i < IQ_FFT; i++) psd[i] = 0.0f;
         nseg = 0;
         for (s = 0; s < IQ_NSEG; s++) {
@@ -437,13 +456,24 @@ void *iq_rx_thr(void *arg)
     for (;;) {
         int wid, idx;
         n = recvfrom(fd, pkt, sizeof(pkt), 0, NULL, NULL);
+        if (!iq_spec_get_enable()) {
+            cur = -1;
+            npkt = 0;
+            continue;
+        }
         if (n < 8 + IQ_PAY) continue;
         if (pkt[0] != 0x49 || pkt[1] != 0x51 || pkt[6] != 0x02) continue;   /* 'IQ', fmt=2 */
         wid = pkt[2];
         idx = ((pkt[4] & 0x03) << 8) | pkt[5];
         if (idx >= IQ_NPKT) continue;
 
-        if (cur < 0) cur = wid;
+        /* After FSSI Refresh is turned off, wait for packet zero so the first
+         * analysed window cannot contain a stale or partial A/B interval. */
+        if (cur < 0) {
+            if (idx != 0) continue;
+            memset(iq_seen[buf], 0, IQ_NPKT);
+            cur = wid;
+        }
         if (wid != cur) {                       /* window boundary: hand this one off */
             int other = buf ^ 1;
             pthread_mutex_lock(&iq_mtx);