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topology-guide, grpc_streams (SUPERSEDED note), the frame-size known-issue (retired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

@@ -13,21 +13,20 @@ namespace ZB.MOM.WW.ScadaBridge.Communication.Actors;
 /// <remarks>
 /// <para>
 /// The actor's receive handlers no longer own the wire plumbing — they capture the current
-/// <c>Sender</c> and hand it to the transport as <paramref name="replyTo"/>. Two implementations
-/// exist behind the <c>ScadaBridge:Communication:CentralTransport</c> flag: the default
-/// <see cref="AkkaCentralTransport"/> (verbatim of the old <c>ClusterClient.Send</c> path,
-/// including the exact sender-forwarding that routes central's reply straight back to the waiting
-/// Ask) and <see cref="Grpc.GrpcCentralTransport"/> (a gRPC dial of <c>CentralControlService</c>).
+/// <c>Sender</c> and hand it to the transport as <paramref name="replyTo"/>. The production
+/// implementation is <see cref="Grpc.GrpcCentralTransport"/> (a gRPC dial of
+/// <c>CentralControlService</c> with sticky central-a→central-b failover) — the only site→central
+/// transport since the ClusterClient→gRPC migration removed the Akka path in Phase 4.
+/// <see cref="NoOpCentralTransport"/> is the fail-loud placeholder used only when the Host injects
+/// nothing (a wiring bug, and in TestKit suites that only exercise command dispatch).
 /// </para>
 /// <para>
-/// <b>Reply/fault contract, identical on both transports.</b> Each Ask-returning method (all but
-/// the heartbeat) guarantees exactly one reply eventually lands at <paramref name="replyTo"/>:
-/// either the real reply type the caller Asks for, or a transient-failure signal. The Akka path
-/// sends a not-accepted ack / <see cref="Status.Failure"/> when no ClusterClient is registered;
-/// the gRPC path sends <see cref="Status.Failure"/> on any non-OK status (a timeout or an
-/// <c>Unavailable</c> that could not be failed over). Both are what the S&amp;F / audit / health
-/// layers above the seam already treat as transient — rows stay buffered, counters restore, the
-/// pass re-runs.
+/// <b>Reply/fault contract.</b> Each Ask-returning method (all but the heartbeat) guarantees exactly
+/// one reply eventually lands at <paramref name="replyTo"/>: either the real reply type the caller
+/// Asks for, or a transient-failure signal. The gRPC path sends <see cref="Status.Failure"/> on any
+/// non-OK status (a timeout or an <c>Unavailable</c> that could not be failed over) — what the
+/// S&amp;F / audit / health layers above the seam already treat as transient (rows stay buffered,
+/// counters restore, the pass re-runs).
 /// </para>
 /// <para>
 /// <b>The heartbeat stays fire-and-forget end-to-end.</b> <see cref="SendHeartbeat"/> takes no