feat(cluster): simultaneous-cold-start split-brain guard (#33)

Port OtOpcUa's bootstrap guard (lmxopcua d1dac87f) to ScadaBridge's
BuildHocon bootstrap. Both pair nodes are self-first seeds, so a true
simultaneous cold start (shared site power event) races FirstSeedNodeProcess
on both and forms two 1-node clusters that never merge.

Opt-in dark switch ScadaBridge:Cluster:BootstrapGuard:Enabled (default off,
guard-off behavior byte-identical). When on: BuildHocon emits an empty seed
list so Akka does not auto-join, and ClusterBootstrapCoordinator (IHostedService,
registered in both the Central and Site composition roots) picks the join order
from ClusterBootstrapGuard's pure decision core — the lower canonical host:port
is the founder (self-first, forms immediately); the higher node TCP-probes the
founder up to PartnerProbeSeconds and joins peer-first if reachable, else
self-first (cold-start-alone preserved). Decides before a single JoinSeedNodes,
never re-forms mid-handshake.

Review notes carried over: case-insensitive founder tie-break; fail-fast
validation of probe timings when enabled; higher-node-cold-start-alone covered
by a real-ActorSystem test. 21 unit + 5 real-cluster tests (incl. the headline
both-cold-start-together-form-one-cluster).

src/ZB.MOM.WW.ScadaBridge.Host/Actors/AkkaHostedService.cs

@@ -261,8 +261,16 @@ public class AkkaHostedService : IHostedService
         TimeSpan transportHeartbeat,
         TimeSpan transportFailure)
     {
-        var seedNodesStr = string.Join(",",
-            clusterOptions.SeedNodes.Select(QuoteHocon));
+        // Simultaneous-cold-start split-brain guard (Gitea #33): when enabled the node must NOT
+        // auto-join from its config seeds — Akka's FirstSeedNodeProcess is exactly what races the
+        // peer and forms two 1-node clusters. Emit an EMPTY seed list so the ActorSystem comes up
+        // unjoined, and ClusterBootstrapCoordinator issues the single reachability-gated
+        // JoinSeedNodes (founder self-first / joiner peer-first). Default OFF, so guard-off configs
+        // render byte-identical to before.
+        var effectiveSeeds = clusterOptions.BootstrapGuard.Enabled
+            ? Enumerable.Empty<string>()
+            : clusterOptions.SeedNodes;
+        var seedNodesStr = string.Join(",", effectiveSeeds.Select(QuoteHocon));
         var rolesStr = string.Join(",", roles.Select(QuoteHocon));
 
         // auto-down (default): AutoDowning provider — the leader among the reachable