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topology-guide, grpc_streams (SUPERSEDED note), the frame-size known-issue (retired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

@@ -143,6 +143,22 @@ public static class StartupValidator
                     + "production as ${secret:SB-GRPC-PSK-<siteId>}) and under the secret "
                     + "name SB-GRPC-PSK-<siteId> in central's secret store");
 
+                // gRPC (CentralControlService) is the only site→central transport after the
+                // ClusterClient→gRPC migration's Phase 4 — the Akka ClusterClient path and its
+                // CentralContactPoints option are gone. A site with no central gRPC endpoint has
+                // nothing to dial: heartbeats, health reports, notification forwards and audit
+                // ingest all silently fail. The shared CommunicationOptionsValidator only rejects
+                // BLANK entries (it is role-agnostic, and central nodes legitimately leave the list
+                // empty), so the "a Site must have at least one" rule lives here, where the role is
+                // known. The predicate reads index :0 directly, so its non-empty presence proves the
+                // list has a usable first endpoint.
+                p.Require("ScadaBridge:Communication:CentralGrpcEndpoints:0",
+                    value => !string.IsNullOrWhiteSpace(value),
+                    "is required for Site nodes: gRPC (CentralControlService) is the only "
+                    + "site→central transport, so each site must list at least one central gRPC "
+                    + "endpoint under ScadaBridge:Communication:CentralGrpcEndpoints "
+                    + "(e.g. http://scadabridge-central-a:8083). Central nodes leave it empty.");
+
                 // ScadaBridge:Database:SiteDbPath was required here until LocalDb
                 // Phase 2. The site's tables now live in the consolidated LocalDb
                 // database (LocalDb:Path, which SiteServiceRegistration requires),