feat(galaxy): debug-log native alarm feed delivery (subs + fanout)

Native-alarm delivery through OnAlarmFeedTransition was a black box — there was no way
to answer 'is the gateway feed delivering / is a subscription un-gating it', which is
partly why the missing-SubscribeAlarmsAsync wiring shipped undetected. Add a single
per-transition Debug line (kind, ref, live subscription count, fanout flag). Debug so a
flapping galaxy doesn't flood prod, but available on demand.

src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.Galaxy/GalaxyDriver.cs

@@ -1128,6 +1128,7 @@ public sealed class GalaxyDriver
     private void OnAlarmFeedTransition(object? sender, GalaxyAlarmTransition transition)
     {
         GalaxyAlarmSubscriptionHandle? handle;
+        int subCount;
         lock (_alarmHandlersLock)
         {
             // Pick the earliest-registered handle as the event owner. The server routes
@@ -1137,10 +1138,17 @@ public sealed class GalaxyDriver
             // subscription remains — HashSet.First() was unstable across mutations
             // (Driver.Galaxy-006 fix). _alarmSubscriptions is a List, so [0] is always
             // the earliest-registered handle.
-            handle = _alarmSubscriptions.Count > 0
-                ? _alarmSubscriptions[0]
-                : null;
+            subCount = _alarmSubscriptions.Count;
+            handle = subCount > 0 ? _alarmSubscriptions[0] : null;
         }
+        // Native-alarm delivery observability (was a black box): one line per feed transition with the
+        // resolved alarm ref, the live subscription count, and whether it fans out to OnAlarmEvent. Debug
+        // so a flapping galaxy doesn't flood prod, but available on demand to answer "is the feed
+        // delivering / is the subscription un-gating it" — the exact question this driver couldn't answer
+        // before the SubscribeAlarmsAsync wiring fix.
+        _logger.LogDebug(
+            "GalaxyDriver alarm feed: {Kind} {Ref} (subs={Subs}, fanout={Fanout})",
+            transition.TransitionKind, transition.AlarmFullReference, subCount, handle is not null);
         if (handle is null) return;
 
         var args = new AlarmEventArgs(