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topology-guide, grpc_streams (SUPERSEDED note), the frame-size known-issue (retired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/HealthReportAckTests.cs

@@ -14,9 +14,9 @@ namespace ZB.MOM.WW.ScadaBridge.Communication.Tests;
 
 /// <summary>
 /// Review 01 [Medium]: site→central health reports were fire-and-forget, so a lost
-/// report was invisible to the sender. These tests pin the new end-to-end ack:
-/// the site actor replies not-accepted when it has no central ClusterClient, and the
-/// central actor processes + acks a report it receives.
+/// report was invisible to the sender. These tests pin the end-to-end ack: the site actor
+/// always replies to the sender (even when the transport cannot forward — a fail-loud failure,
+/// not silence), and the central actor processes + acks a report it receives.
 /// </summary>
 public class HealthReportAckTests : TestKit
 {
@@ -29,17 +29,17 @@ public class HealthReportAckTests : TestKit
             0, 0, new Dictionary<string, int>(), 0, 0, 0, 0);
 
     [Fact]
-    public void SiteCommunicationActor_NoCentralClient_RepliesNotAccepted()
+    public void SiteCommunicationActor_NoTransport_RepliesFailure_NotSilence()
     {
+        // With no transport injected the actor falls back to the fail-loud
+        // NoOpCentralTransport, which answers a Status.Failure so the health transport's
+        // Ask sees a transient failure rather than hanging. (Production always injects the
+        // gRPC GrpcCentralTransport; this pins the wiring-guard fallback.)
         var dmProbe = CreateTestProbe();
         var siteComm = Sys.ActorOf(Props.Create(() =>
             new SiteCommunicationActor("site-a", _options, dmProbe.Ref)));
-        // No RegisterCentralClient sent => _centralClient is null.
         siteComm.Tell(SampleReport(seq: 7));
-        var ack = ExpectMsg<SiteHealthReportAck>();
-        Assert.False(ack.Accepted);
-        Assert.Equal(7, ack.SequenceNumber);
-        Assert.Equal("site-a", ack.SiteId);
+        ExpectMsg<Status.Failure>();
     }
 
     [Fact]
@@ -54,8 +54,8 @@ public class HealthReportAckTests : TestKit
         services.AddSingleton(aggregator);
         var sp = services.BuildServiceProvider();
 
-        var siteClientFactory = Substitute.For<ISiteClientFactory>();
-        var actor = Sys.ActorOf(Props.Create(() => new CentralCommunicationActor(sp, siteClientFactory)));
+        var transport = Substitute.For<ISiteCommandTransport>();
+        var actor = Sys.ActorOf(Props.Create(() => new CentralCommunicationActor(sp, transport, (TimeSpan?)null)));
 
         actor.Tell(SampleReport(seq: 3));
         var ack = ExpectMsg<SiteHealthReportAck>();