fix(cluster-infrastructure): resolve ClusterInfrastructure-009,010 — DownIfAlone consumption (via Host-012), validator enforces DownIfAlone=true

src/ScadaLink.ClusterInfrastructure/ClusterOptionsValidator.cs

@@ -65,6 +65,15 @@ public sealed class ClusterOptionsValidator : IValidateOptions<ClusterOptions>
                 "declared unreachable before a heartbeat can arrive.");
         }
 
+        if (!options.DownIfAlone)
+        {
+            failures.Add(
+                "ClusterOptions.DownIfAlone must be true for the keep-oldest resolver "
+                + "(Component-ClusterInfrastructure.md → Split-Brain Resolution); with it false the "
+                + "oldest node can run as an isolated single-node cluster during a partition while the "
+                + "younger node forms its own, producing two live clusters.");
+        }
+
         return failures.Count > 0
             ? ValidateOptionsResult.Fail(failures)
             : ValidateOptionsResult.Success;