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topology-guide, grpc_streams (SUPERSEDED note), the frame-size known-issue (retired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

@@ -189,8 +189,9 @@ public class SiteActorPathTests : IAsyncLifetime
                 // fixture actually starts the host — a site node with no configured
                 // consolidated-database path fails fast rather than booting half-working.
                 ["LocalDb:Path"] = _tempLocalDbPath,
-                // Configure a dummy central contact point to trigger ClusterClient creation
-                ["ScadaBridge:Communication:CentralContactPoints:0"] = "akka.tcp://scadabridge@localhost:25510",
+                // A central gRPC endpoint so the Site branch builds its GrpcCentralTransport
+                // (the only site→central transport since Phase 4). Never dialled by this test.
+                ["ScadaBridge:Communication:CentralGrpcEndpoints:0"] = "http://localhost:8083",
             });
         });
         builder.ConfigureServices((context, services) =>
@@ -236,9 +237,10 @@ public class SiteActorPathTests : IAsyncLifetime
     public async Task SiteActors_SiteCommunication_Exists()
         => await AssertActorExists("/user/site-communication");
 
-    [Fact]
-    public async Task SiteActors_CentralClusterClient_Exists()
-        => await AssertActorExists("/user/central-cluster-client");
+    // SiteActors_CentralClusterClient_Exists was removed with the site→central Akka ClusterClient
+    // (the `central-cluster-client` actor) in the ClusterClient→gRPC migration's Phase 4. The site
+    // now reaches central over gRPC (GrpcCentralTransport dialling CentralControlService), which is
+    // not an actor in the local system, so there is no actor path to assert here.
 
     private async Task AssertActorExists(string path)
     {