feat: add CLI debug snapshot command for one-shot instance state inspection

Adds `debug snapshot --id <int>` to query a running instance's current
attribute values and alarm states without the subscribe/stream overhead
of the debug view. Routes through ManagementActor → CommunicationService
→ site DeploymentManager → InstanceActor using the existing remote query
pattern.

src/ScadaLink.SiteRuntime/Actors/InstanceActor.cs

@@ -133,6 +133,9 @@ public class InstanceActor : ReceiveActor
         Receive<SubscribeDebugViewRequest>(HandleSubscribeDebugView);
         Receive<UnsubscribeDebugViewRequest>(HandleUnsubscribeDebugView);
 
+        // Debug snapshot (one-shot, no subscription)
+        Receive<DebugSnapshotRequest>(HandleDebugSnapshot);
+
         // Handle internal messages
         Receive<LoadOverridesResult>(HandleOverridesLoaded);
     }
@@ -399,6 +402,35 @@ public class InstanceActor : ReceiveActor
             _instanceUniqueName, request.CorrelationId);
     }
 
+    /// <summary>
+    /// One-shot debug snapshot — returns current state without registering a subscriber.
+    /// </summary>
+    private void HandleDebugSnapshot(DebugSnapshotRequest request)
+    {
+        var attributeValues = _attributes.Select(kvp => new AttributeValueChanged(
+            _instanceUniqueName,
+            kvp.Key,
+            kvp.Key,
+            kvp.Value,
+            _attributeQualities.GetValueOrDefault(kvp.Key, "Good"),
+            DateTimeOffset.UtcNow)).ToList();
+
+        var alarmStates = _alarmStates.Select(kvp => new AlarmStateChanged(
+            _instanceUniqueName,
+            kvp.Key,
+            kvp.Value,
+            0,
+            DateTimeOffset.UtcNow)).ToList();
+
+        var snapshot = new DebugViewSnapshot(
+            _instanceUniqueName,
+            attributeValues,
+            alarmStates,
+            DateTimeOffset.UtcNow);
+
+        Sender.Tell(snapshot);
+    }
+
     /// <summary>
     /// Publishes attribute change to stream and notifies child Script/Alarm actors.
     /// WP-22: Tell for attribute notifications (fire-and-forget, never blocks).