docs(grpc): Phase 2 live gate — PASS; site→central S&F cutover soak

All 6 site nodes flipped to CentralTransport=Grpc; whole control plane
(heartbeat/health/notification S&F/audit) rides gRPC CentralControlService,
196 RPCs/90s 0 non-200, 0 PermissionDenied, 0 health-sequence regressions.

Soak driven by a live SoakNotify workload (3 instances, 3 notifs/5s):
- single-node active-kill (central-a): sticky failover central-a:8083→b:8083
  logged instantly, central-b active in 29s, buffer drained 72→101, every 5s
  bucket = exactly 3 through the gap, 101==101 distinct
- failback: central-a rejoined ready ~5s as standby, central-b kept active
  (oldest-Up, no flap), traffic uninterrupted
- full outage (both central down ~59s): count frozen, cold re-form central-b
  active ~14s, ~42 buffered drained, every 5s bucket = exactly 3 across the
  whole dead window, 216==216 distinct — zero loss, zero dupes

Also previews Phase 5 checks 4 (failover/failback) and 5 (mid-drain kill).
Rig config reverted to Akka default (defaults stay Akka until Phase 4).

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

@@ -198,7 +198,7 @@
       "id": "P2",
       "phase": "2",
       "subject": "All sites CentralTransport=Grpc; central-kill S&F soak, failback observed, health sequences clean",
-      "status": "pending",
+      "status": "completed",
       "activeForm": "Running the site->central cutover soak",
       "blockedBy": [
         "P1A.DoD"