refactor(mesh): step 4 — drop the receptionist extension and akka.cluster.client from akka.conf

akka.extensions loses ClusterClientReceptionistExtensionProvider and the whole
akka.cluster.client block (buffer-size / reconnect-timeout / receptionist.role) goes with
it. The DistributedPubSub extension and the maximum-frame-size / log-frame-size-exceeding
canary both stay — DPS is the pair-local transport and the frame canary guards every
remoting message, neither of which the cross-mesh deletion touches.

MeshTransportHoconTests drops its three ClusterClientSettings/ClusterReceptionistSettings
asserts and keeps the frame-size pin and the DPS-extension pin, renamed since it no longer
has a receptionist to survive alongside.

src/Core/ZB.MOM.WW.OtOpcUa.Cluster/Resources/akka.conf

@@ -13,14 +13,12 @@ akka {
     # since the Serilog logger can be slow to initialize at startup.
     logger-startup-timeout = 30s
 
+    # DistributedPubSub is PAIR-LOCAL and stays: it carries MeshTransport:Mode = Dps (the
+    # single-mesh default), the node-local alarm-command publishes and the node<->node
+    # redundancy-state probe leg. What is gone is the cross-mesh command transport, which is now
+    # gRPC and needs no Akka extension at all.
     extensions = [
         "Akka.Cluster.Tools.PublishSubscribe.DistributedPubSubExtensionProvider, Akka.Cluster.Tools"
-
-        # Per-cluster mesh Phase 2 — the central<->node command boundary. NOT auto-started: without
-        # this entry the receptionist materialises only if some code path happens to call
-        # ClusterClientReceptionist.Get(system) first, which makes "is the boundary listening?"
-        # depend on startup ordering rather than on configuration.
-        "Akka.Cluster.Tools.Client.ClusterClientReceptionistExtensionProvider, Akka.Cluster.Tools"
     ]
 
     actor {
@@ -94,31 +92,6 @@ akka {
         down-removal-margin = 15s
         run-coordinated-shutdown-when-down = on
 
-        # CLUSTERCLIENT — the central<->node command boundary (per-cluster mesh Phase 2).
-        client {
-            # DELIBERATE, and NOT what the sister project runs: they never wrote this section, so
-            # they inherit buffer-size = 1000. Zero means "if the receptionist is unknown, drop the
-            # message now". Buffering would replay a DispatchDeployment on reconnect and apply a
-            # deployment ConfigPublishCoordinator had already sealed as TimedOut — the node would
-            # then be running a configuration the database says failed. Fail-fast keeps the DB the
-            # single record of truth, and reproduces today's DPS behaviour exactly: a node that is
-            # down misses the publish and self-heals from the DB on restart.
-            buffer-size = 0
-
-            # Retry forever rather than self-stopping the client. Restates Akka's default; stated
-            # explicitly because the behaviour is load-bearing — a central node down for an hour
-            # must not require a driver-node restart to become reachable again.
-            reconnect-timeout = off
-
-            receptionist {
-                # Empty = every member hosts a receptionist. This is what makes the comm actors
-                # "registered per node, not as a singleton" work: contact rotation reaches whichever
-                # node answers. Restates Akka's default; do NOT scope this to a role, or the
-                # boundary becomes reachable through one node only and rotation silently fails.
-                role = ""
-            }
-        }
-
         singleton {
             singleton-name = "singleton"
         }