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topology-guide, grpc_streams (SUPERSEDED note), the frame-size known-issue (retired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

@@ -66,18 +66,15 @@ public sealed class CommunicationOptionsValidator : OptionsValidatorBase<Communi
         builder.RequireThat(options.GrpcMaxConcurrentStreams > 0,
             $"ScadaBridge:Communication:GrpcMaxConcurrentStreams must be positive (was {options.GrpcMaxConcurrentStreams}).");
 
-        // The gRPC site→central transport needs at least one central endpoint to dial. Only
-        // enforced when that transport is selected — the default Akka path ignores the list, so a
-        // node on ClusterClient must not be forced to declare gRPC endpoints it never uses.
-        if (options.CentralTransport == CentralTransportMode.Grpc)
-        {
-            builder.RequireThat(
-                options.CentralGrpcEndpoints.Count > 0
-                    && options.CentralGrpcEndpoints.All(e => !string.IsNullOrWhiteSpace(e)),
-                "ScadaBridge:Communication:CentralGrpcEndpoints must list at least one non-empty "
-                + "central gRPC endpoint when CentralTransport is Grpc "
-                + $"(was {options.CentralGrpcEndpoints.Count} entr{(options.CentralGrpcEndpoints.Count == 1 ? "y" : "ies")}).");
-        }
+        // The gRPC site→central transport needs at least one central endpoint to dial. gRPC is now
+        // the only site→central transport (ClusterClient was removed in the migration's Phase 4), so
+        // every site node must declare its central endpoints — there is no Akka fallback to ignore
+        // the list. Central nodes leave it empty (they host CentralControlService, they don't dial it).
+        builder.RequireThat(
+            options.CentralGrpcEndpoints.All(e => !string.IsNullOrWhiteSpace(e)),
+            "ScadaBridge:Communication:CentralGrpcEndpoints must not contain empty or whitespace "
+            + "entries (a site node must list at least one central gRPC endpoint; central nodes "
+            + "leave it empty).");
 
         // ── Aggregated live alarm cache (plan #10, Task 6) ───────────────────────
         // Linger drives a Timer dueTime; TimeSpan.Zero is valid (stop the aggregator