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.Host/Grpc/NodeCommandGrpcService.cs

@@ -15,11 +15,11 @@ using Status = Grpc.Core.Status;
 namespace ZB.MOM.WW.OtOpcUa.Host.Grpc;
 
 /// <summary>
-///     Node-side command ingress (ClusterClient→gRPC migration Phase 1). Central dials a driver node
+///     Node-side command ingress (mesh gRPC command plane, Phase 1). Central dials a driver node
 ///     and calls <see cref="Execute"/>; the envelope is decoded back into the same CLR record the
-///     ClusterClient receptionist used to deliver and handed to this node's
+///     node-local DistributedPubSub path delivers and handed to this node's
 ///     <c>NodeCommunicationActor</c>, which re-emits it on the node's <c>EventStream</c> exactly as
-///     before. Nothing downstream of that actor can tell which transport carried the command.
+///     it would otherwise. Nothing downstream of that actor can tell which transport carried it.
 /// </summary>
 /// <remarks>
 ///     <para>