feat(comm): Phase 4 — delete Akka ClusterClient site↔central transport, gRPC-only

ClusterClient→gRPC migration Phase 4 (docs/plans/2026-07-22-clusterclient-to-grpc-plan.md).
Phases 2/3 proved both directions on gRPC; this removes the Akka transport underneath.

Deleted:
- AkkaCentralTransport, AkkaSiteTransport (+ their dedicated tests)
- ISiteClientFactory + DefaultSiteClientFactory; CentralCommunicationActor legacy
  ctor + SelectTransport (Host now builds GrpcSiteTransport and injects it)
- ClusterClient creation + both ClusterClientReceptionist.RegisterService calls in
  AkkaHostedService; the RegisterCentralClient message + receive block
- CommunicationOptions.CentralContactPoints; the CentralTransport/SiteTransport
  coexistence flags; the CentralTransportMode/SiteTransportKind enums

gRPC is now the only site↔central transport (site→central CentralControlService via
GrpcCentralTransport; central→site SiteCommandService via GrpcSiteTransport), both
built unconditionally by the Host. NoOpCentralTransport is the fail-loud null-default
so TestKit command-dispatch suites still construct the site actor without a wired
transport; production always injects GrpcCentralTransport.

Config: CentralGrpcEndpoints is now unconditional — CommunicationOptionsValidator
rejects blank entries (role-agnostic), and StartupValidator requires a Site node to
list >=1 endpoint (fail-fast, mirrors GrpcPsk). Rig configs moved
CentralContactPoints -> CentralGrpcEndpoints (docker x6, docker-env2 x2, Host default,
deploy/wonder-app-vd03). Kept Akka.Cluster.Tools (ClusterSingleton still used).

Tests: build 0/0; Communication.Tests 640, Host.Tests 421 green. Removed the
ClusterClient.Send per-site-routing tests (covered by the transport suites), swapped
the ISiteClientFactory-based ctors to a substitute ISiteCommandTransport, converted
the audit-push integration relay to an in-process bridge transport.

Docs: Component-Communication/Host/StoreAndForward, components/Communication,
topology-guide, grpc_streams (SUPERSEDED note), the frame-size known-issue (retired
amendment), and CLAUDE.md transport decisions.

Not included: the dead IntegrationCallRequest path (#32) is a separate user-owned
behavioral decision — SiteEnvelope routing is transport-agnostic so it still compiles.

tests/ZB.MOM.WW.ScadaBridge.Communication.Tests/CoordinatorSupervisionTests.cs

@@ -12,7 +12,7 @@ namespace ZB.MOM.WW.ScadaBridge.Communication.Tests;
 /// explicit <c>Resume</c> supervision strategy per the CLAUDE.md decision
 /// ("Resume for coordinator actors"). A child fault under the default
 /// (Restart) strategy would wipe a child's in-memory state; the long-lived
-/// coordinators own per-site ClusterClients and must not silently discard
+/// coordinators own per-site transport resources and must not silently discard
 /// their children on a transient fault.
 /// </summary>
 public class CoordinatorSupervisionTests : TestKit
@@ -23,8 +23,8 @@ public class CoordinatorSupervisionTests : TestKit
     /// </summary>
     private sealed class CentralCommunicationActorProbe : CentralCommunicationActor
     {
-        public CentralCommunicationActorProbe(IServiceProvider sp, ISiteClientFactory factory)
-            : base(sp, factory) { }
+        public CentralCommunicationActorProbe(IServiceProvider sp, ISiteCommandTransport transport)
+            : base(sp, transport) { }
 
         public SupervisorStrategy GetSupervisorStrategy() => SupervisorStrategy();
     }
@@ -54,10 +54,10 @@ public class CoordinatorSupervisionTests : TestKit
     public void CentralCommunicationActor_SupervisorStrategy_IsResume()
     {
         var sp = EmptyServiceProvider();
-        var factory = Substitute.For<ISiteClientFactory>();
+        var transport = Substitute.For<ISiteCommandTransport>();
 
         var actorRef = new Akka.TestKit.TestActorRef<CentralCommunicationActorProbe>(
-            Sys, Props.Create(() => new CentralCommunicationActorProbe(sp, factory)));
+            Sys, Props.Create(() => new CentralCommunicationActorProbe(sp, transport)));
 
         var strategy = actorRef.UnderlyingActor.GetSupervisorStrategy();