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topology-guide, grpc_streams (SUPERSEDED note), the frame-size known-issue (retired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

@@ -105,26 +105,28 @@ public class CommunicationOptionsValidatorTests
         Assert.Contains("LiveAlarmCachePublishCoalesce", result.FailureMessage);
     }
 
-    // ── T1A.3: gRPC central transport endpoints (required only when selected) ────
+    // ── gRPC central transport endpoints ─────────────────────────────────────────
+    // gRPC (CentralControlService) is the only site→central transport after the
+    // ClusterClient→gRPC migration's Phase 4. This role-agnostic validator only rejects BLANK
+    // entries; an EMPTY list is valid (central nodes legitimately declare none). The role-aware
+    // "a Site must list at least one endpoint" rule lives in StartupValidator, tested there.
 
     [Fact]
-    public void GrpcTransport_WithNoEndpoints_IsRejected()
+    public void EmptyEndpoints_IsValid()
     {
+        // A central node hosts CentralControlService; it does not dial it, so it declares none.
         var result = Validate(new CommunicationOptions
         {
-            CentralTransport = CentralTransportMode.Grpc,
             CentralGrpcEndpoints = new List<string>(),
         });
-        Assert.True(result.Failed);
-        Assert.Contains("CentralGrpcEndpoints", result.FailureMessage);
+        Assert.True(result.Succeeded, result.FailureMessage);
     }
 
     [Fact]
-    public void GrpcTransport_WithBlankEndpoint_IsRejected()
+    public void BlankEndpoint_IsRejected()
     {
         var result = Validate(new CommunicationOptions
         {
-            CentralTransport = CentralTransportMode.Grpc,
             CentralGrpcEndpoints = new List<string> { "   " },
         });
         Assert.True(result.Failed);
@@ -132,11 +134,10 @@ public class CommunicationOptionsValidatorTests
     }
 
     [Fact]
-    public void GrpcTransport_WithEndpoints_IsValid()
+    public void Endpoints_IsValid()
     {
         var result = Validate(new CommunicationOptions
         {
-            CentralTransport = CentralTransportMode.Grpc,
             CentralGrpcEndpoints = new List<string>
             {
                 "http://scadabridge-central-a:8083",
@@ -145,16 +146,4 @@ public class CommunicationOptionsValidatorTests
         });
         Assert.True(result.Succeeded, result.FailureMessage);
     }
-
-    [Fact]
-    public void AkkaTransport_IgnoresMissingGrpcEndpoints()
-    {
-        // The default transport must not be forced to declare gRPC endpoints it never dials.
-        var result = Validate(new CommunicationOptions
-        {
-            CentralTransport = CentralTransportMode.Akka,
-            CentralGrpcEndpoints = new List<string>(),
-        });
-        Assert.True(result.Succeeded, result.FailureMessage);
-    }
 }