fix: add event-id latch to linked monitor score detection

Linked monitor alarm detection was purely point-in-time: it only
returned a score when the linked monitor's state was exactly ALARM at
the instant of the check. If the linked monitor's entire alarm cycle
(ALARM -> ALERT -> IDLE) occurred between two analysis cycles of the
checking monitor, the alarm was missed entirely.

Add a latch using last_event_id: if the linked monitor's event id has
changed since the last check, a new event started and we return a
score even if the ALARM state has already passed. Also remove dead
code in hasAlarmed() where last_event_id was updated after an early
return.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/zm_monitor_monitorlink.cpp

@@ -218,6 +218,19 @@ int Monitor::MonitorLink::score() {
     Debug(1, "Checking zone %u, zone_index is %d, score is %d", zone_id, zone_index, zone_scores[zone_index]);
     return zone_scores[zone_index];
   }
+
+  // Latch: detect if a new event started since we last checked.
+  // This catches alarms even if the linked monitor transitioned through
+  // ALARM -> ALERT -> IDLE between our analysis cycles.
+  if (shared_data->last_event_id != last_event_id) {
+    Debug(1, "New event %" PRIu64 " on linked monitor (was %" PRIu64 "), last_frame_score %d",
+          shared_data->last_event_id, last_event_id, shared_data->last_frame_score);
+    last_event_id = shared_data->last_event_id;
+    // Return last_frame_score if still available, otherwise minimum of 1
+    // to ensure the alarm is not silently missed
+    return shared_data->last_frame_score > 0 ? shared_data->last_frame_score : 1;
+  }
+
   if (shared_data->state == ALARM) {
     Debug(1, "Checking all zones score is %d", shared_data->last_frame_score);
     return shared_data->last_frame_score;
@@ -228,6 +241,4 @@ int Monitor::MonitorLink::score() {
 
 bool Monitor::MonitorLink::hasAlarmed() {
   return this->score() > 0;
-  last_event_id = shared_data->last_event_id;
-  return false;
 }