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topology-guide, grpc_streams (SUPERSEDED note), the frame-size known-issue (retired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

@@ -41,6 +41,9 @@ public class StartupValidatorTests
         // T0.3: the gRPC control plane is fail-closed, so a Site node without a preshared
         // key serves nothing while still looking healthy. Required at boot for that reason.
         ["ScadaBridge:Communication:GrpcPsk"] = "test-site-control-plane-key",
+        // Phase 4: gRPC (CentralControlService) is the only site→central transport, so a Site
+        // node must list at least one central gRPC endpoint to dial (no Akka fallback remains).
+        ["ScadaBridge:Communication:CentralGrpcEndpoints:0"] = "http://central-a:8083",
     };
 
     [Fact]
@@ -97,6 +100,30 @@ public class StartupValidatorTests
         Assert.Null(Record.Exception(() => StartupValidator.Validate(config)));
     }
 
+    [Fact]
+    public void SiteWithoutCentralGrpcEndpoints_FailsValidation()
+    {
+        // Phase 4: gRPC (CentralControlService) is the only site→central transport, so a Site
+        // node with no central gRPC endpoint has nothing to dial — heartbeats, health, notification
+        // forwards and audit ingest all silently fail. Fail fast at boot instead.
+        var values = ValidSiteConfig();
+        values.Remove("ScadaBridge:Communication:CentralGrpcEndpoints:0");
+        var config = BuildConfig(values);
+
+        var ex = Assert.Throws<InvalidOperationException>(() => StartupValidator.Validate(config));
+        Assert.Contains("CentralGrpcEndpoints", ex.Message);
+    }
+
+    [Fact]
+    public void CentralWithoutCentralGrpcEndpoints_PassesValidation()
+    {
+        // A central node hosts CentralControlService; it does not dial it, so it declares no
+        // endpoints. The site-only requirement must not fire for Central.
+        var config = BuildConfig(ValidCentralConfig());
+
+        Assert.Null(Record.Exception(() => StartupValidator.Validate(config)));
+    }
+
     [Fact]
     public void MissingRole_FailsValidation()
     {