fix(grpc): typed alarm-feed overflow, memoized alarm snapshot, cancellable stream writes, uniform reply correlation id

- StreamAlarms: slow subscribers are completed with
  AlarmFeedSubscriberOverflowException and surface as ResourceExhausted
  (like a StreamEvents overflow) instead of warning-with-stack + Unavailable.
- GatewayAlarmMonitor.StreamAsync reuses the memoized alarm-set projection
  instead of cloning the whole set under the broadcast lock per attach; the
  prefix filter runs outside the lock.
- StreamEvents/StreamAlarms pass the call token to response writes; a write
  failure after client cancel maps to Cancelled without a warning.
- Every MxCommandReply.correlation_id now echoes the request's
  client_correlation_id (previously only constraint-denied replies did;
  normal replies leaked the internal envelope id).

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

@@ -967,7 +967,9 @@ public sealed class GatewayAlarmMonitor : BackgroundService, IGatewayAlarmServic
     }
 
     // Caller holds _sync. Pushes a feed message to every matching subscriber;
-    // a subscriber that has fallen behind is completed with an error and dropped.
+    // a subscriber that has fallen behind is completed with an AlarmFeedSubscriberOverflowException
+    // and dropped. The typed fault is what lets StreamAlarms answer ResourceExhausted instead of
+    // treating a slow client as a gateway failure.
     private void Broadcast(AlarmFeedMessage message, string reference)
     {
         for (int index = _subscribers.Count - 1; index >= 0; index--)
@@ -980,8 +982,7 @@ public sealed class GatewayAlarmMonitor : BackgroundService, IGatewayAlarmServic
 
             if (!subscriber.Channel.Writer.TryWrite(message))
             {
-                subscriber.Channel.Writer.TryComplete(new InvalidOperationException(
-                    "Alarm feed subscriber fell behind and was dropped; reconnect to re-snapshot."));
+                subscriber.Channel.Writer.TryComplete(new AlarmFeedSubscriberOverflowException());
                 _subscribers.RemoveAt(index);
             }
         }
@@ -997,8 +998,7 @@ public sealed class GatewayAlarmMonitor : BackgroundService, IGatewayAlarmServic
             Subscriber subscriber = _subscribers[index];
             if (!subscriber.Channel.Writer.TryWrite(message))
             {
-                subscriber.Channel.Writer.TryComplete(new InvalidOperationException(
-                    "Alarm feed subscriber fell behind and was dropped; reconnect to re-snapshot."));
+                subscriber.Channel.Writer.TryComplete(new AlarmFeedSubscriberOverflowException());
                 _subscribers.RemoveAt(index);
             }
         }
@@ -1046,9 +1046,14 @@ public sealed class GatewayAlarmMonitor : BackgroundService, IGatewayAlarmServic
             _subscribers.Add(subscriber);
             providerStatus = BuildProviderStatus();
             snapshotTruncated = _snapshotTruncated;
-            snapshot = _alarms.Values
-                .Where(alarm => prefix.Length == 0
-                    || alarm.AlarmFullReference.StartsWith(prefix, StringComparison.Ordinal))
+
+            // Take the memoized projection rather than cloning the whole set per attach: the
+            // clone runs at most once per alarm-set generation (shared with CurrentAlarms), so a
+            // burst of reconnecting subscribers no longer holds the broadcast lock for one full
+            // copy each. The array is never mutated once built — a write to _alarms replaces it
+            // with null — so capturing the reference here pins exactly the generation this
+            // subscriber registered against, which is what keeps snapshot-then-live ordering.
+            snapshot = _currentAlarmsProjection ??= _alarms.Values
                 .Select(alarm => alarm.Clone())
                 .ToArray();
         }
@@ -1068,8 +1073,15 @@ public sealed class GatewayAlarmMonitor : BackgroundService, IGatewayAlarmServic
                 SnapshotStatus = new AlarmSnapshotStatus { Truncated = snapshotTruncated },
             };
 
+            // The prefix filter runs here, outside _sync, over the immutable projection.
             foreach (ActiveAlarmSnapshot alarm in snapshot)
             {
+                if (prefix.Length != 0
+                    && !alarm.AlarmFullReference.StartsWith(prefix, StringComparison.Ordinal))
+                {
+                    continue;
+                }
+
                 yield return new AlarmFeedMessage { ActiveAlarm = alarm };
             }