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.

tests/Server/ZB.MOM.WW.OtOpcUa.Runtime.Tests/Communication/GrpcApplyAckSenderTests.cs

@@ -13,21 +13,21 @@ using ZB.MOM.WW.OtOpcUa.Runtime.Communication;
 namespace ZB.MOM.WW.OtOpcUa.Runtime.Tests.Communication;
 
 /// <summary>
-///     The node-side gRPC ack sender (ClusterClient→gRPC migration Task 1B.2). The client is a fake,
+///     The node-side gRPC ack sender (mesh gRPC command plane, Task 1B.2). The client is a fake,
 ///     so what is under test is the sender's own logic: the ordered single-pass over
 ///     <c>CentralGrpcEndpoints</c>, the per-call credentials, and the drop that used to be silent.
 /// </summary>
 /// <remarks>
 ///     <para>
 ///         <b>The load-bearing fact is that ONE endpoint is dialled on success.</b> This replaces an
-///         Akka <c>ClusterClient.Send</c>, which is single-target at-most-once — not a
+///         one ack per node per dispatch, single-target at-most-once — not a
 ///         <c>SendToAll</c>. Both central nodes route an ack to the same
 ///         <c>ConfigPublishCoordinator</c> singleton proxy, whose NodeId-keyed upsert absorbs a
 ///         duplicate idempotently — so the single-endpoint rule is defence in depth and a bound
 ///         on wasted dials, not what protects the seal.
 ///     </para>
 ///     <para>
-///         The failure side matters just as much: the receptionist dropped an undeliverable ack in
+///         The failure side matters just as much: an undeliverable ack is dropped in
 ///         silence, and the whole point of the migration is that the same drop is now a Warning naming
 ///         every endpoint and the status it answered with.
 ///     </para>