perf(alarms): memoize CurrentAlarms projection, invalidate on mutation

src/ZB.MOM.WW.MxGateway.Server/Alarms/GatewayAlarmMonitor.cs

@@ -34,6 +34,13 @@ public sealed class GatewayAlarmMonitor : BackgroundService, IGatewayAlarmServic
     private readonly Dictionary<string, ActiveAlarmSnapshot> _alarms = new(StringComparer.Ordinal);
     private readonly List<Subscriber> _subscribers = [];
 
+    // Memoized CurrentAlarms projection, guarded by _sync: the cloned, read-only view of _alarms
+    // handed to the dashboard and the QueryActiveAlarms RPC. Cloning the whole set per read held
+    // _sync — the broadcast lock — for the length of the copy, so a polled dashboard stalled every
+    // ApplyTransition/Broadcast behind it. Null means "not built for the current generation":
+    // every path that writes _alarms must null this under _sync, or readers keep a stale set.
+    private ActiveAlarmSnapshot[]? _currentAlarmsProjection;
+
     // NEXT-03 dedup tombstones, guarded by _sync: alarm instances whose Clear was synthesized by
     // the most recent reconcile pass, keyed by reference with the instance's original raise
     // timestamp as the identity marker. A buffered live Clear for the same instance is a duplicate
@@ -93,7 +100,12 @@ public sealed class GatewayAlarmMonitor : BackgroundService, IGatewayAlarmServic
         {
             lock (_sync)
             {
-                return _alarms.Values.Select(alarm => alarm.Clone()).ToArray();
+                // Same clone semantics as an uncached read — callers still get instances no
+                // mutation can leak back into the cache — but built once per alarm-set
+                // generation instead of once per caller.
+                return _currentAlarmsProjection ??= _alarms.Values
+                    .Select(alarm => alarm.Clone())
+                    .ToArray();
             }
         }
     }
@@ -422,6 +434,11 @@ public sealed class GatewayAlarmMonitor : BackgroundService, IGatewayAlarmServic
             if (transition.TransitionKind == AlarmTransitionKind.Clear)
             {
                 bool wasKnown = _alarms.Remove(reference);
+                if (wasKnown)
+                {
+                    _currentAlarmsProjection = null;
+                }
+
                 if (!wasKnown && IsDuplicateOfReconcileClear(reference, transition))
                 {
                     return;
@@ -433,6 +450,7 @@ public sealed class GatewayAlarmMonitor : BackgroundService, IGatewayAlarmServic
                 bool duplicate = _alarms.TryGetValue(reference, out ActiveAlarmSnapshot? existing)
                     && IsDuplicateOfCachedState(existing, snapshot);
                 _alarms[reference] = snapshot;
+                _currentAlarmsProjection = null;
                 if (duplicate)
                 {
                     return;
@@ -650,6 +668,8 @@ public sealed class GatewayAlarmMonitor : BackgroundService, IGatewayAlarmServic
             {
                 _alarms[incoming.Key] = incoming.Value;
             }
+
+            _currentAlarmsProjection = null;
         }
     }
 
@@ -696,6 +716,7 @@ public sealed class GatewayAlarmMonitor : BackgroundService, IGatewayAlarmServic
         lock (_sync)
         {
             _alarms.Clear();
+            _currentAlarmsProjection = null;
         }
     }