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).

tests/Server/ZB.MOM.WW.OtOpcUa.Host.IntegrationTests/ActorSystemTerminationWatchdogTests.cs

@@ -0,0 +1,92 @@
+using Akka.Actor;
+using Microsoft.Extensions.Hosting;
+using Microsoft.Extensions.Logging.Abstractions;
+using Shouldly;
+using Xunit;
+using ZB.MOM.WW.OtOpcUa.Host;
+
+namespace ZB.MOM.WW.OtOpcUa.Host.IntegrationTests;
+
+/// <summary>
+///     Unit coverage for the #459 down-if-alone recovery watchdog: an unexpected
+///     <see cref="ActorSystem"/> termination (SBR self-down) must stop the host so the supervisor
+///     restarts it, while a graceful host shutdown must stay silent and not re-trigger stop.
+/// </summary>
+[Trait("Category", "Unit")]
+public sealed class ActorSystemTerminationWatchdogTests
+{
+    /// <summary>An unexpected ActorSystem termination stops the host so the supervisor restarts the node.</summary>
+    [Fact]
+    public async Task Unexpected_termination_stops_the_application()
+    {
+        var system = ActorSystem.Create("watchdog-unexpected");
+        var lifetime = new FakeHostApplicationLifetime();
+        var watchdog = new ActorSystemTerminationWatchdog(
+            () => system, lifetime, NullLogger<ActorSystemTerminationWatchdog>.Instance);
+
+        await watchdog.StartAsync(CancellationToken.None);
+
+        // Simulate the SBR self-down: the ActorSystem terminates while the host is still running.
+        await system.Terminate();
+
+        await WaitForAsync(() => lifetime.StopApplicationCalls > 0, TimeSpan.FromSeconds(5));
+        lifetime.StopApplicationCalls.ShouldBe(1, "an out-of-band ActorSystem termination must stop the host once");
+    }
+
+    /// <summary>A graceful host stop (StopAsync ran) leaves the termination continuation silent.</summary>
+    [Fact]
+    public async Task Graceful_stop_does_not_stop_the_application()
+    {
+        var system = ActorSystem.Create("watchdog-graceful");
+        var lifetime = new FakeHostApplicationLifetime();
+        var watchdog = new ActorSystemTerminationWatchdog(
+            () => system, lifetime, NullLogger<ActorSystemTerminationWatchdog>.Instance);
+
+        await watchdog.StartAsync(CancellationToken.None);
+
+        // Host is shutting down normally: StopAsync runs, THEN the system terminates.
+        await watchdog.StopAsync(CancellationToken.None);
+        await system.Terminate();
+
+        // Give any continuation a chance to run, then assert it stayed silent.
+        await Task.Delay(500, TestContext.Current.CancellationToken);
+        lifetime.StopApplicationCalls.ShouldBe(0, "a graceful shutdown must not re-trigger StopApplication");
+    }
+
+    /// <summary>Termination while the host is already stopping (ApplicationStopping fired) stays silent.</summary>
+    [Fact]
+    public async Task Termination_while_already_stopping_stays_silent()
+    {
+        var system = ActorSystem.Create("watchdog-already-stopping");
+        var lifetime = new FakeHostApplicationLifetime();
+        var watchdog = new ActorSystemTerminationWatchdog(
+            () => system, lifetime, NullLogger<ActorSystemTerminationWatchdog>.Instance);
+
+        await watchdog.StartAsync(CancellationToken.None);
+
+        lifetime.TriggerStopping(); // the host is already tearing down (ApplicationStopping cancelled)
+        await system.Terminate();
+
+        await Task.Delay(500, TestContext.Current.CancellationToken);
+        lifetime.StopApplicationCalls.ShouldBe(0, "termination during an in-progress host shutdown must stay silent");
+    }
+
+    private static async Task WaitForAsync(Func<bool> condition, TimeSpan timeout)
+    {
+        var deadline = DateTime.UtcNow + timeout;
+        while (!condition() && DateTime.UtcNow < deadline)
+            await Task.Delay(20);
+    }
+
+    /// <summary>Minimal <see cref="IHostApplicationLifetime"/> that records <see cref="StopApplication"/> calls.</summary>
+    private sealed class FakeHostApplicationLifetime : IHostApplicationLifetime
+    {
+        private readonly CancellationTokenSource _stopping = new();
+        public int StopApplicationCalls { get; private set; }
+        public CancellationToken ApplicationStarted => CancellationToken.None;
+        public CancellationToken ApplicationStopping => _stopping.Token;
+        public CancellationToken ApplicationStopped => CancellationToken.None;
+        public void StopApplication() => StopApplicationCalls++;
+        public void TriggerStopping() => _stopping.Cancel();
+    }
+}