test(mesh): step 7 — replace the boundary test with MeshGrpcBoundaryTests and sweep the comments

MeshGrpcBoundaryTests replaces the MeshClusterClientBoundaryTests deleted in step 1. Three
self-seeded single-member clusters sharing the ActorSystem name, joined ONLY by gRPC, driving
the REAL CentralCommunicationActor -> MeshGrpcCommandDialer -> Kestrel h2c ->
MeshCommandAuthInterceptor -> NodeCommandGrpcService -> NodeCommunicationActor -> EventStream:

  1. No_cross_mesh_Akka_association_forms_between_central_and_either_site — the program's own
     point, kept as a test rather than a live-gate note: each cluster stays single-member and
     none ever holds another's address as a member.
  2. One_dispatch_through_central_reaches_a_node_in_every_cluster_mesh — the fan, with each
     site holding its OWN per-cluster PSK, so a dial that selected the wrong cluster's key
     would be refused at the far side.
  3. Falsifiability_control_a_dial_carrying_the_wrong_cluster_key_delivers_nothing — without
     it, (2) cannot distinguish "the gRPC boundary carried it" from "something else in the
     process did". The same node then accepts the right key, so the silence is the auth gate
     and not a dead endpoint.

All three run fully offline — no Docker fixture — and pass.

Comment sweep across every remaining clusterclient|receptionist hit in src/ and tests/:
the program label becomes "mesh gRPC command plane", and every comment that described the
deleted transport as live now describes what actually runs (Dps = pair-local default,
Grpc = cross-mesh). mesh_transport.proto's header/service comments are reworded; no message,
field or enum is touched — the contract is locked. MeshCommActorPathTests is reshaped rather
than retired: the two paths still exist and still matter, but what a mismatch costs is now a
misleading Unavailable from the gRPC ingress rather than a silently dropped receptionist send.

The only surviving mentions of the old names are the ones that must survive: the fail-closed
rules that name "ClusterClient" as a retired value refused at boot, their tests, and this
test class's own account of what it replaces.

src/Server/ZB.MOM.WW.OtOpcUa.Runtime/Drivers/DriverHostActor.cs

@@ -374,9 +374,8 @@ public sealed class DriverHostActor : ReceiveActor, IWithTimers
     /// <param name="dbFactory">Database context factory for configuration database access.</param>
     /// <param name="localNode">The local cluster node identifier.</param>
     /// <param name="ackRouter">Where ApplyAcks are sent. A direct coordinator handle in tests; under
-    /// <c>MeshTransport:Mode = ClusterClient</c> <b>and</b> <c>= Grpc</c> the node's
-    /// <c>NodeCommunicationActor</c>, which relays across the boundary over whichever transport it was
-    /// built with. Null publishes on the <c>deployment-acks</c> DPS topic.</param>
+    /// <c>MeshTransport:Mode = Grpc</c> the node's <c>NodeCommunicationActor</c>, which relays the ack
+    /// across the mesh boundary. Null publishes on the <c>deployment-acks</c> DPS topic.</param>
     /// <param name="driverFactory">Optional driver factory; defaults to null factory if not provided.</param>
     /// <param name="localRoles">Optional set of roles assigned to the local node.</param>
     /// <param name="dependencyMux">Optional actor reference for dependency multiplexing.</param>
@@ -459,9 +458,8 @@ public sealed class DriverHostActor : ReceiveActor, IWithTimers
     /// <param name="dbFactory">Database context factory for configuration database access.</param>
     /// <param name="localNode">The local cluster node identifier.</param>
     /// <param name="ackRouter">Where ApplyAcks are sent. A direct coordinator handle in tests; under
-    /// <c>MeshTransport:Mode = ClusterClient</c> <b>and</b> <c>= Grpc</c> the node's
-    /// <c>NodeCommunicationActor</c>, which relays across the boundary over whichever transport it was
-    /// built with. Null publishes on the <c>deployment-acks</c> DPS topic.</param>
+    /// <c>MeshTransport:Mode = Grpc</c> the node's <c>NodeCommunicationActor</c>, which relays the ack
+    /// across the mesh boundary. Null publishes on the <c>deployment-acks</c> DPS topic.</param>
     /// <param name="driverFactory">Optional driver factory; defaults to null factory if not provided.</param>
     /// <param name="localRoles">Optional set of roles assigned to the local node.</param>
     /// <param name="dependencyMux">Optional actor reference for dependency multiplexing.</param>
@@ -584,11 +582,11 @@ public sealed class DriverHostActor : ReceiveActor, IWithTimers
         _mediator.Tell(
             new Subscribe(ZB.MOM.WW.OtOpcUa.Runtime.OpcUa.OpcUaPublishActor.RedundancyStateTopic, Self));
 
-        // Per-cluster mesh Phase 2 dark switch. Under MeshTransport:Mode = ClusterClient these same
-        // commands arrive from NodeCommunicationActor on the node-local EventStream instead of the
-        // mesh-wide DPS topic. Subscribing to BOTH unconditionally is deliberate: exactly one of the
-        // two ever publishes, so the transport flag lives entirely on the publishing side and the
-        // switch can be flipped (or reverted) without touching any subscriber.
+        // Per-cluster mesh Phase 2. Under MeshTransport:Mode = Grpc these same commands arrive from
+        // NodeCommunicationActor on the node-local EventStream instead of the mesh-wide DPS topic.
+        // Subscribing to BOTH unconditionally is deliberate: exactly one of the two ever publishes, so
+        // the transport choice lives entirely on the publishing side and the mode can be flipped (or
+        // reverted) without touching any subscriber.
         Context.System.EventStream.Subscribe(Self, typeof(DispatchDeployment));
         Context.System.EventStream.Subscribe(Self, typeof(RestartDriver));
         Context.System.EventStream.Subscribe(Self, typeof(ReconnectDriver));
@@ -2492,11 +2490,11 @@ public sealed class DriverHostActor : ReceiveActor, IWithTimers
         var ack = new ApplyAck(deploymentId, _localNode, outcome, failureReason, correlation);
         if (_ackRouter is not null)
         {
-            // Either a direct coordinator handle (tests) or, under MeshTransport:Mode = ClusterClient
-            // and = Grpc alike, the NodeCommunicationActor that relays this across the boundary (it
-            // owns the choice of transport, not this actor). Renamed from _coordinatorOverride in
-            // per-cluster mesh Phase 2: under those two modes this ref is emphatically NOT a
-            // coordinator, and the old name would send the next reader looking for one.
+            // Either a direct coordinator handle (tests) or, under MeshTransport:Mode = Grpc, the
+            // NodeCommunicationActor that relays this across the mesh boundary (it owns the transport,
+            // not this actor). Renamed from _coordinatorOverride in per-cluster mesh Phase 2: under
+            // Grpc this ref is emphatically NOT a coordinator, and the old name would send the next
+            // reader looking for one.
             _ackRouter.Tell(ack);
         }
         else