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topology-guide, grpc_streams (SUPERSEDED note), the frame-size known-issue (retired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

src/ZB.MOM.WW.ScadaBridge.Communication/Actors/SiteCommunicationActor.cs

@@ -51,13 +51,14 @@ public class SiteCommunicationActor : ReceiveActor, IWithTimers
     private readonly SiteCommandDispatcher _dispatcher;
 
     /// <summary>
-    /// The site→central transport. Finalized in <see cref="PreStart"/> to the injected instance,
-    /// or a default <see cref="AkkaCentralTransport"/> (ClusterClient) when none is supplied — so
-    /// the seven site→central sends delegate here rather than owning the wire plumbing inline.
+    /// The site→central transport. Finalized in <see cref="PreStart"/> to the injected instance, or a
+    /// fail-loud <see cref="NoOpCentralTransport"/> when none is supplied — so the seven site→central
+    /// sends delegate here rather than owning the wire plumbing inline. Production injects the gRPC
+    /// <see cref="Grpc.GrpcCentralTransport"/>.
     /// </summary>
     private ICentralTransport _transport;
 
-    /// <summary>The transport supplied by the Host (null selects the default Akka transport).</summary>
+    /// <summary>The transport supplied by the Host (null falls back to <see cref="NoOpCentralTransport"/>).</summary>
     private readonly ICentralTransport? _injectedTransport;
 
     /// <summary>
@@ -81,10 +82,9 @@ public class SiteCommunicationActor : ReceiveActor, IWithTimers
     /// ActorSystem.
     /// </param>
     /// <param name="transport">
-    /// The site→central transport. <c>null</c> (the default, used by every existing test and by
-    /// the Host's Akka path) selects an <see cref="AkkaCentralTransport"/> over ClusterClient; the
-    /// Host injects a <see cref="Grpc.GrpcCentralTransport"/> when
-    /// <c>ScadaBridge:Communication:CentralTransport</c> is <c>Grpc</c>.
+    /// The site→central transport. Production always passes the Host-built gRPC
+    /// <see cref="Grpc.GrpcCentralTransport"/>. <c>null</c> (used by TestKit suites that only
+    /// exercise command dispatch) falls back to a fail-loud <see cref="NoOpCentralTransport"/>.
     /// </param>
     /// <param name="dispatcher">
     /// The shared <see cref="SiteCommandDispatcher"/> (production: created by the Host and also
@@ -121,13 +121,6 @@ public class SiteCommunicationActor : ReceiveActor, IWithTimers
                 ClusterState.ClusterFailoverCoordinator.FailOverOldest(system, role, dryRun)?.ToString();
         _dispatcher = dispatcher ?? new SiteCommandDispatcher(siteId, deploymentManagerProxy, resolveFailover);
 
-        // Registration. Feeding the ClusterClient into the transport is a no-op unless the
-        // default/Akka transport is in use — the gRPC transport dials configured endpoints and
-        // never receives this message (the Host does not create a ClusterClient for it).
-        Receive<RegisterCentralClient>(msg =>
-        {
-            (_transport as AkkaCentralTransport)?.SetCentralClient(msg.Client);
-        });
         Receive<RegisterLocalHandler>(HandleRegisterLocalHandler);
 
         // ── The 27 migrated central→site commands (28th is failover, below) all route
@@ -234,11 +227,11 @@ public class SiteCommunicationActor : ReceiveActor, IWithTimers
     /// <inheritdoc />
     protected override void PreStart()
     {
-        // Finalize the transport now that the actor context (and _log) exist. The default Akka
-        // transport is given this actor's logging adapter so the "no ClusterClient registered"
-        // warnings are preserved exactly. PreStart always runs before any message, so the Receive
-        // closures above see a non-null _transport.
-        _transport = _injectedTransport ?? new AkkaCentralTransport(_log);
+        // Finalize the transport now that the actor context (and _log) exist. When the Host injects
+        // none, fall back to a fail-loud NoOpCentralTransport (given this actor's logging adapter so
+        // "no transport configured" warnings surface). PreStart always runs before any message, so
+        // the Receive closures above see a non-null _transport.
+        _transport = _injectedTransport ?? new NoOpCentralTransport(_log);
 
         _log.Info("SiteCommunicationActor started for site {0}", _siteId);
 
@@ -362,11 +355,6 @@ public class SiteCommunicationActor : ReceiveActor, IWithTimers
     internal record SendHeartbeat;
 }
 
-/// <summary>
-/// Command to register a ClusterClient for communicating with the central cluster.
-/// </summary>
-public record RegisterCentralClient(IActorRef Client);
-
 /// <summary>
 /// Command to register a local actor as a handler for a specific message pattern.
 /// </summary>