feat(comm): T1B.3 — central-side gRPC site-command transport seam (default Akka)

Extract the central→site send path in CentralCommunicationActor behind a new
ISiteCommandTransport, selected by ScadaBridge:Communication:SiteTransport
(Akka | Grpc, default Akka — rollback = flip the flag). CommunicationService's
27 commands, SiteCallAuditActor's 2 parked relays and DebugStreamBridgeActor's
subscribe/unsubscribe are untouched; the seam sits below SiteEnvelope.

- AkkaSiteTransport: today's per-site ClusterClient path extracted verbatim
  (the _siteClients lookup + ClusterClient.Send with the reply-to sender
  preserved, and the "no client ⇒ warn + drop, caller's Ask times out" path).
- GrpcSiteTransport: dials the site SiteCommandService (T1B.1 proto client) via
  SiteCommandDtoMapper, PSK + x-scadabridge-site on the channel through
  ControlPlaneCredentials, per-command deadlines set EQUAL to today's
  CommunicationService Ask timeouts (per-command, not per-group: DeploymentState
  query and TriggerSiteFailover use QueryTimeout; the two parked relays map to
  QueryTimeout so SiteCallAudit's inner RelayTimeout 10s < 30s ordering holds;
  WaitForAttribute keeps its dynamic Timeout + IntegrationTimeout).
- SitePairChannelProvider: per-site A/B channel pair with sticky failover
  (flip only on Unavailable — NEVER on DeadlineExceeded, a write/deploy/failover
  may have run), background failback probe to the preferred node with 1s→60s
  doubling backoff, PSK invalidation on site removal. Fed by the SAME DB refresh
  loop (extended to carry GrpcNodeA/GrpcNodeBAddress) — no second poll.

Tests: actor-with-substitute-transport (routing, Ask-reply plumbing, per-site
lifecycle across refreshes), ResolveDeadline pinned to each command's current
Ask timeout, and GrpcSiteTransport/SitePairChannelProvider over dual in-process
TestServers (PSK+header+deadline attached, Unavailable failover + stickiness,
failback to preferred, no-retry-on-DeadlineExceeded). Proto csproj untouched
(no active <Protobuf> item). Full solution builds 0 warnings; Communication.Tests
607 green with Akka default.

src/ZB.MOM.WW.ScadaBridge.Communication/ZB.MOM.WW.ScadaBridge.Communication.csproj

@@ -10,6 +10,9 @@
   <ItemGroup>
     <InternalsVisibleTo Include="ZB.MOM.WW.ScadaBridge.Communication.Tests" />
     <InternalsVisibleTo Include="ZB.MOM.WW.ScadaBridge.IntegrationTests" />
+    <!-- GrpcSiteTransport failover/failback TestServer coverage (T1B.3) lives in Host.Tests,
+         which owns the ASP.NET TestHost stack; it drives the provider's internal failback seams. -->
+    <InternalsVisibleTo Include="ZB.MOM.WW.ScadaBridge.Host.Tests" />
   </ItemGroup>
 
   <ItemGroup>