feat(cluster): AllowSingleNodeCluster flag — single-seed installs drop the phantom seed

Validator lowers the seed-count floor to 1 when AllowSingleNodeCluster is set,
otherwise still requires 2 and points operators at the flag. The plan's
deploy/wonder-app-vd03/appsettings.Central.json edit is skipped: that production
deploy artifact is not tracked in this repo (only its deployment record doc is).

src/ZB.MOM.WW.ScadaBridge.ClusterInfrastructure/ClusterOptions.cs

@@ -76,4 +76,12 @@ public class ClusterOptions
     /// other reachable members, rather than running as an isolated single-node cluster.
     /// </summary>
     public bool DownIfAlone { get; set; } = true;
+
+    /// <summary>
+    /// Acknowledges a deliberate single-node deployment: permits exactly one seed
+    /// node (normally ≥2 are required so either node can start first). Without this
+    /// flag a single-node install must list a phantom second seed, which the node
+    /// dials forever — log noise and a defeated validation intent (review 01).
+    /// </summary>
+    public bool AllowSingleNodeCluster { get; set; }
 }

src/ZB.MOM.WW.ScadaBridge.ClusterInfrastructure/ClusterOptionsValidator.cs

@@ -26,11 +26,13 @@ public sealed class ClusterOptionsValidator : OptionsValidatorBase<ClusterOption
         // two. Accepting a single-seed configuration silently defeats the
         // "no startup ordering dependency" guarantee called out by
         // Component-ClusterInfrastructure.md (Node Configuration).
-        builder.RequireThat(options.SeedNodes is not null && options.SeedNodes.Count >= 2,
-            "ClusterOptions.SeedNodes must contain at least 2 seed nodes "
-            + "(Component-ClusterInfrastructure.md → Node Configuration: "
-            + "both nodes are seed nodes); a single-seed configuration defeats "
-            + "the no-startup-ordering-dependency guarantee.");
+        var minSeeds = options.AllowSingleNodeCluster ? 1 : 2;
+        builder.RequireThat(options.SeedNodes is not null && options.SeedNodes.Count >= minSeeds,
+            options.AllowSingleNodeCluster
+                ? "ClusterOptions.SeedNodes must contain at least 1 seed node."
+                : "ClusterOptions.SeedNodes must contain at least 2 seed nodes "
+                  + "(Component-ClusterInfrastructure.md → Node Configuration: both nodes are seed nodes); "
+                  + "for a deliberate single-node install set ClusterOptions.AllowSingleNodeCluster = true instead of listing a phantom seed.");
 
         builder.RequireThat(
             !string.IsNullOrWhiteSpace(options.SplitBrainResolverStrategy)

tests/ZB.MOM.WW.ScadaBridge.ClusterInfrastructure.Tests/ClusterOptionsValidatorTests.cs

@@ -69,6 +69,47 @@ public class ClusterOptionsValidatorTests
         Assert.Contains("SeedNodes", result.FailureMessage);
     }
 
+    [Fact]
+    public void SingleSeed_WithoutAcknowledgement_Fails()
+    {
+        var options = ValidOptions();
+        options.SeedNodes = new List<string> { "akka.tcp://scadabridge@h:8081" };
+
+        var result = new ClusterOptionsValidator().Validate(null, options);
+
+        Assert.True(result.Failed);
+        Assert.Contains("AllowSingleNodeCluster", result.FailureMessage);
+    }
+
+    [Fact]
+    public void SingleSeed_WithAllowSingleNodeCluster_Passes()
+    {
+        // Review 01 [Low]: the shipped single-node artifact satisfied the 2-seed
+        // rule with a phantom seed the node dials forever. An explicit
+        // acknowledgement flag replaces the fiction.
+        var options = ValidOptions();
+        options.SeedNodes = new List<string> { "akka.tcp://scadabridge@h:8081" };
+        options.AllowSingleNodeCluster = true;
+
+        var result = new ClusterOptionsValidator().Validate(null, options);
+
+        Assert.True(result.Succeeded, result.FailureMessage);
+    }
+
+    [Fact]
+    public void EmptySeeds_FailsEvenWithFlag()
+    {
+        // Zero seeds is always invalid — the flag lowers the floor to 1, not 0.
+        var options = ValidOptions();
+        options.SeedNodes = new List<string>();
+        options.AllowSingleNodeCluster = true;
+
+        var result = new ClusterOptionsValidator().Validate(null, options);
+
+        Assert.True(result.Failed);
+        Assert.Contains("SeedNodes", result.FailureMessage);
+    }
+
     [Fact]
     public void SingleSeedNode_FailsValidation()
     {