Expose monotonic age of verified substream progress

src/stream/stream_metrics.c

@@ -104,6 +104,7 @@ int stream_metrics_poll(struct stream_metrics *m, uint64_t now_ms, struct stream
     metrics_detect_stop(m, now_ms);
     result.lag_ms = m->lag_ms;
     result.expected_interval_ms = m->expected_interval_ms;
+    result.progress_age_ms = now_ms - m->last_receive_ms;
     result.stopped = m->stopped;
     if (!m->ended && now_ms - m->last_receive_ms > m->expected_interval_ms)
         result.lag_ms = metrics_add(result.lag_ms, now_ms - m->last_receive_ms - m->expected_interval_ms);

src/stream/stream_metrics.h

@@ -45,6 +45,7 @@ struct stream_metric_snapshot {
     uint64_t source_rate_bytes_sec;
     uint64_t receive_rate_bytes_sec;
     uint64_t expected_interval_ms;
+    uint64_t progress_age_ms; /* Монотонный возраст последнего реально принятого блока; известен при успешном poll. */
     int stopped;
     int source_rate_valid;
     int receive_rate_valid;