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topology-guide, grpc_streams (SUPERSEDED note), the frame-size known-issue (retired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

@@ -1,51 +1,16 @@
 namespace ZB.MOM.WW.ScadaBridge.Communication;
 
-/// Which transport carries the seven site→central control messages. Selected per node by
-/// <c>ScadaBridge:Communication:CentralTransport</c>; the migration ships with
-/// <see cref="Akka"/> as the default so nothing flips until a node opts in.
-/// </summary>
-public enum CentralTransportMode
-{
-    /// <summary>Akka <c>ClusterClient</c> — the transport in production today, and the default.</summary>
-    Akka = 0,
-
-    /// <summary>gRPC dial of the central <c>CentralControlService</c> (Phase 1A migration target).</summary>
-    Grpc = 1,
-}
-
-/// <summary>
-/// Selects the transport the central→site command plane rides on. The Akka
-/// per-site <c>ClusterClient</c> path is the default until the gRPC cutover
-/// (ClusterClient→gRPC migration, Phase 1B); flipping to <see cref="Grpc"/> is the
-/// rollback-by-flag switch.
-/// </summary>
-public enum SiteTransportKind
-{
-    /// <summary>Route <c>SiteEnvelope</c>s through the per-site Akka <c>ClusterClient</c> (today's default).</summary>
-    Akka,
-
-    /// <summary>Route <c>SiteEnvelope</c>s over the site <c>SiteCommandService</c> gRPC plane.</summary>
-    Grpc
-}
-
 /// <summary>
 /// Configuration options for central-site communication, including per-pattern
 /// timeouts and transport heartbeat settings.
 /// </summary>
 public class CommunicationOptions
 {
-    /// <summary>
-    /// Which transport carries the site→central control messages. Default <see cref="CentralTransportMode.Akka"/>
-    /// (ClusterClient) — coexistence rule: a node flips to gRPC only by setting this to <c>Grpc</c>,
-    /// and rollback is flipping it back. Selecting <c>Grpc</c> requires <see cref="CentralGrpcEndpoints"/>.
-    /// </summary>
-    public CentralTransportMode CentralTransport { get; set; } = CentralTransportMode.Akka;
-
     /// <summary>
     /// Central control-plane gRPC endpoints (preferred first), e.g.
     /// <c>["http://scadabridge-central-a:8083", "http://scadabridge-central-b:8083"]</c>. Dialled by
-    /// <see cref="Grpc.CentralChannelProvider"/> with sticky failover/failback. Required when
-    /// <see cref="CentralTransport"/> is <see cref="CentralTransportMode.Grpc"/>, ignored otherwise.
+    /// <see cref="Grpc.CentralChannelProvider"/> with sticky failover/failback. Required on every site
+    /// node — gRPC (<c>CentralControlService</c>) is the only site→central transport.
     /// </summary>
     /// <remarks>
     /// Sites reach central by container/host name, NOT via Traefik (which is HTTP/1 only; gRPC is
@@ -53,15 +18,6 @@ public class CommunicationOptions
     /// </remarks>
     public List<string> CentralGrpcEndpoints { get; set; } = new();
 
-    /// <summary>
-    /// Which transport the central→site command plane uses. Default <see cref="SiteTransportKind.Akka"/>
-    /// (the per-site ClusterClient path) — flipping to <see cref="SiteTransportKind.Grpc"/> moves
-    /// every <c>SiteEnvelope</c> onto the site <c>SiteCommandService</c> gRPC plane. Selected inside
-    /// <c>CentralCommunicationActor</c>; <c>CommunicationService</c> and <c>SiteCallAuditActor</c>
-    /// are unchanged either way. Rollback at any point = flip this back to <c>Akka</c>.
-    /// </summary>
-    public SiteTransportKind SiteTransport { get; set; } = SiteTransportKind.Akka;
-
     /// <summary>Timeout for deployment commands (typically longest due to apply logic).</summary>
     public TimeSpan DeploymentTimeout { get; set; } = TimeSpan.FromMinutes(2);
 
@@ -91,12 +47,6 @@ public class CommunicationOptions
     /// </summary>
     public TimeSpan NotificationForwardTimeout { get; set; } = TimeSpan.FromSeconds(30);
 
-    /// <summary>
-    /// Contact point addresses for the central cluster (e.g. "akka.tcp://scadabridge@central-a:8081").
-    /// Used by site nodes to create a ClusterClient for reaching central.
-    /// </summary>
-    public List<string> CentralContactPoints { get; set; } = new();
-
     /// <summary>
     /// Preshared key authenticating this node's gRPC control plane — the site↔central
     /// boundary. On a site node this is the key its inbound gate