fix(communication): failback probe sentinel uses an illegal actor name so it can never match a real instance (review 2026-10-21 03/P-2 follow-up)

src/ZB.MOM.WW.ScadaBridge.Communication/Grpc/SitePairChannelProvider.cs

@@ -589,8 +589,14 @@ public sealed class SitePairChannelProvider : IDisposable
         return GrpcChannel.ForAddress(endpoint, channelOptions);
     }
 
-    /// <summary>Sentinel instance name the readiness probe queries; never a real instance.</summary>
-    internal const string ReadinessProbeInstance = "__scadabridge-failback-probe__";
+    /// <summary>
+    /// Sentinel instance name the readiness probe queries. The leading <c>$</c> makes it an
+    /// illegal Akka actor name, which <c>ActorNameRules</c> rejects for every real instance, so it
+    /// can never collide with one. The site-side DeploymentStateQuery path (dispatcher →
+    /// DeploymentManagerActor → a parameterised SQLite lookup) never validates or materialises
+    /// the name, so the probe simply reads "not deployed".
+    /// </summary>
+    internal const string ReadinessProbeInstance = "$scadabridge-failback-probe";
 
     /// <summary>
     /// Production failback probe (review 2026-10-21 03/P-2): a real, read-only