docs(grpc): Phase 3 live gate — PASS; central→site command cutover

Central flipped to SiteTransport=Grpc (both nodes, central-wide flag), sites
kept on gRPC from Phase 2 → both directions on gRPC at once. Central logs
'central→site command transport: gRPC (SiteCommandService)'; 0 ClusterClient-to-
site on either central.

Command matrix over SiteCommandService (all 200):
- ExecuteQuery (event-log) + ExecuteParked (parked) — all 3 sites
- ExecuteLifecycle disable/enable #95 on site-a — NEW live coverage vs 1B/P2
  (SoakNotify instances survived the recreate as Enabled)

Resilience:
- hard-kill the ACTIVE site-a node mid-query-loop → in-flight call returned
  TIMEOUT at exactly the 30s QueryTimeout deadline (bounded, no hang; deadline≠
  retry — an in-flight call can't be safely re-sent)
- next + all subsequent queries recovered automatically via site-a-b
  (SitePairChannelProvider NodeA→NodeB); site→central S&F never stopped
  (notif 619→715, still no dupes)
- 0 PermissionDenied across all 8 nodes

ExecuteOpcUa/ExecuteRoute/TriggerFailover deferred (no OPC-bound instance / no
CLI verb; unit-proven). Rig left both-gRPC; git reverted to Akka default.

docs/plans/2026-07-22-clusterclient-to-grpc-plan.md.tasks.json

@@ -208,7 +208,7 @@
       "id": "P3",
       "phase": "3",
       "subject": "Central SiteTransport=Grpc all sites; full UI command matrix; site-kill mid-command clean; zero ClusterClient activity",
-      "status": "pending",
+      "status": "completed",
       "activeForm": "Running the central->site cutover soak",
       "blockedBy": [
         "P1B.DoD"