fix(redundancy): 2-node SBR exit-and-rejoin recovery — watchdog + restart supervision + both-node seeds (#459)

Corrects the #459 finding. 2-node keep-oldest recovery works fine (the ScadaBridge
sister project proves it); OtOpcUa was missing the supervision pieces that make it
automatic, and docs/Redundancy.md wrongly claimed in-place oldest-crash failover.

Mechanism (confirmed on a 2-container rig + by decompiling Akka KeepOldest.OldestDecision):
on an OLDEST-node crash keep-oldest downs the LONE survivor (DownReachable including
myself) — down-if-alone can't rescue a lone survivor (its branch needs >=2 survivors).
Recovery is exit-and-rejoin: run-coordinated-shutdown-when-down terminates the node and
the service supervisor restarts it. My earlier 'total outage' was a docker-dev artifact
(no restart policy); production Install-Services.ps1 already has sc.exe failure restart.

Changes (ScadaBridge parity):
- ActorSystemTerminationWatchdog (Host, registered after AddAkka): watches
  ActorSystem.WhenTerminated and on an unexpected self-down calls StopApplication so the
  process exits (supervisor restarts it) instead of idling with a dead actor system.
  Distinguishes graceful shutdown via _stopRequested + ApplicationStopping. 3 unit tests.
- docker-dev: restart: unless-stopped on the host anchor (models production supervision) +
  both redundancy peers in SeedNodes so a restarted node re-forms via either peer.
- docs/Redundancy.md: rewrote the split-brain recovery section — younger-loss = in-place
  fast failover; oldest-loss = exit-and-rejoin under supervision (not in-place); the three
  requirements (supervisor + watchdog + both-node seeds); flagged HardKillFailoverTests as
  non-representative (Transport.Shutdown, not a real crash). Instant in-place takeover on
  ANY single loss needs 3+ members.

Cluster.Tests 29/29 (SBR guards), watchdog tests 3/3, full solution builds.
Live re-verify of the watchdog image pending (host docker disk full).

src/Server/ZB.MOM.WW.OtOpcUa.Host/Program.cs

@@ -296,6 +296,16 @@ builder.Services.AddAkka("otopcua", (ab, sp) =>
         ab.WithOtOpcUaRuntimeActors();
 });
 
+// Down-if-alone recovery watchdog (#459). Registered AFTER AddAkka so it starts after Akka's own
+// hosted service has built the ActorSystem; it resolves the system lazily (never at construction) so
+// it can't race startup. On an unexpected SBR self-down it stops the host so the service supervisor
+// (sc.exe failure / docker restart: unless-stopped) restarts this node — the 2-node keep-oldest
+// exit-and-rejoin recovery path.
+builder.Services.AddHostedService(sp => new ActorSystemTerminationWatchdog(
+    () => sp.GetRequiredService<ActorSystem>(),
+    sp.GetRequiredService<IHostApplicationLifetime>(),
+    sp.GetRequiredService<ILogger<ActorSystemTerminationWatchdog>>()));
+
 if (hasAdmin)
 {
     // Auth + AdminUI surface only mounted on admin-role nodes. Driver-only nodes have no UI.