feat(cluster): self-first seed ordering closes the boot-alone outage gap

Every node now lists ITSELF as seed-nodes[0] and its partner second. Akka runs
FirstSeedNodeProcess -- the only bootstrap path that can form a NEW cluster when
no peer answers InitJoin -- exclusively for seed-nodes[0]; every other node runs
JoinSeedNodeProcess and retries InitJoin forever. That is why a lone cold-starting
central-b never came Up (the "registered outage gap"), and self-first ordering
closes it using Akka's own protocol.

- 6 node appsettings swapped (the *-node-b configs; the -a nodes were already
  self-first). All 14 shipped node configs now satisfy the invariant.
- StartupValidator enforces it at boot, comparing host AND port -- the invariant
  fails silently when broken, so it is enforced loudly. NOTE: the gitignored
  deploy/wonder-app-vd03/ overlay must be reordered before its next deploy or
  that node will refuse to boot.
- SelfFirstSeedBootstrapTests: real in-process clusters at production
  failure-detection timings, incl. a falsifiability control proving the OLD
  peer-first ordering never forms.

Rejected alternative (implemented, measured, discarded): an external self-form
timer calling Cluster.Join(SelfAddress) after a window. It sits outside Akka's
join handshake and so cannot tell "no seed answered" from "a seed answered and
the join is in flight". On a routine standby restart the peer is alive but the
join stalls behind removal of the node's own stale incarnation; a Join(self)
during TryingToJoin abandons the in-flight join and forms a second cluster at
the same address -- still split after 90s. Docs that claimed self-first ordering
was unsafe for simultaneous cold start are corrected: while mutually reachable
the InitJoin handshake converges them to one cluster (measured).

src/ZB.MOM.WW.ScadaBridge.Host/StartupValidator.cs

@@ -93,6 +93,22 @@ public static class StartupValidator
             .Require("ScadaBridge:Cluster:SeedNodes",
                 _ => seedNodes != null && seedNodes.Count >= 2,
                 "must have at least 2 entries")
+            // Self-first seed ordering (decision 2026-07-22). Akka runs FirstSeedNodeProcess —
+            // the ONLY bootstrap path that can form a new cluster when no peer answers InitJoin —
+            // exclusively when seed-nodes[0] is this node's own address. Every other node runs
+            // JoinSeedNodeProcess and retries InitJoin forever, so a node listing its partner
+            // first cannot cold-start alone: that is the "registered outage gap"
+            // (docker/README.md), and it is a silent failure at boot rather than a loud one.
+            // Enforced here rather than in ClusterOptionsValidator because only this validator
+            // sees both the node identity and the seed list.
+            .Require("ScadaBridge:Cluster:SeedNodes",
+                _ => seedNodes is not { Count: >= 1 }
+                     || SeedNodeIsSelf(seedNodes[0], nodeSection["NodeHostname"], port),
+                "must list this node itself first: seed-nodes[0] has to be this node's own "
+                + $"'akka.tcp://scadabridge@{nodeSection["NodeHostname"]}:{port}'. Akka only lets "
+                + "seed-nodes[0] form a new cluster, so with the partner listed first this node "
+                + "can never start while its peer is down (Component-ClusterInfrastructure.md → "
+                + "Seed Node Ordering)")
             // The big Site-only block: GrpcPort/MetricsPort validity + cross-field
             // collisions + seed-node-port loop, in the original order.
             .When(role == "Site", p =>
@@ -155,4 +171,33 @@ public static class StartupValidator
 
         return int.TryParse(seedNode[(lastColon + 1)..], out var port) ? port : -1;
     }
+
+    /// <summary>
+    /// Extracts the host from an Akka seed-node address of the form
+    /// <c>akka.tcp://system@host:port</c>. Returns an empty string when no host can be parsed.
+    /// </summary>
+    private static string SeedNodeHost(string seedNode)
+    {
+        if (string.IsNullOrWhiteSpace(seedNode))
+            return string.Empty;
+
+        var at = seedNode.LastIndexOf('@');
+        var lastColon = seedNode.LastIndexOf(':');
+        if (at < 0 || lastColon <= at)
+            return string.Empty;
+
+        return seedNode[(at + 1)..lastColon];
+    }
+
+    /// <summary>
+    /// True when <paramref name="seedNode"/> addresses this node itself (host AND port).
+    /// Host comparison is case-insensitive because DNS names are; it is otherwise exact —
+    /// Akka does no DNS canonicalisation either, so <c>node-a</c> and
+    /// <c>node-a.example.com</c> are genuinely different seed identities to the cluster.
+    /// </summary>
+    private static bool SeedNodeIsSelf(string seedNode, string? nodeHostname, int remotingPort)
+    {
+        return SeedNodePort(seedNode) == remotingPort
+            && string.Equals(SeedNodeHost(seedNode), nodeHostname, StringComparison.OrdinalIgnoreCase);
+    }
 }