feat(cluster): simultaneous-cold-start split-brain guard (opt-in)

Closes the residual Phase-7 gap: when BOTH nodes of a 2-node pair cold-start at the
same instant, each self-first seed runs FirstSeedNodeProcess, times out waiting for
the other, and forms its own 1-node cluster — two Primaries in one pair (the Phase 6
live gate reproduced this reliably on docker). The prior mitigation was operational
(staggered start / compose depends_on), which does not exist on production hardware.

The guard (dark switch Cluster:BootstrapGuard:Enabled, default OFF) prevents the split
without giving up cold-start-alone:
- The lower-address node is the preferred founder: self-first, forms immediately.
- The higher node probes its partner's Akka port (TCP connect) up to PartnerProbeSeconds:
  reachable => peer-first (join the founder, never race it); unreachable => self-first
  (partner is dead, form alone). The order is decided BEFORE the single JoinSeedNodes,
  from an explicit reachability signal — never re-formed mid-handshake (the retired
  SelfFormAfter failure mode). When on, Akka gets no config seeds (BuildClusterOptions)
  and ClusterBootstrapCoordinator drives the join.

Review-driven hardening: case-insensitive tie-break (a hostname-casing mismatch would
reopen the split); fail-fast validation of the timing knobs; the residual "founder dies
in the probe->join window" hang is documented and made operator-visible (warning + a
restart recovers). Real-ActorSystem coordinator tests cover the load-bearing higher-node
cold-start-alone case; 147/147 Cluster tests pass.

Live-gated on docker-dev (site-a = enablement demo, serialization removed, guard on;
site-b keeps depends_on-serialization + guard off as the A/B control): simultaneous
start -> site-a-1 founds, site-a-2 probes-reachable-joins -> 250/240, NO split;
higher-node-alone -> probes dead founder, self-forms -> 250; founder rejoin -> 240.

Claude-Session: https://claude.ai/code/session_01GASWkNEi68FSCtvr6rLoEW

src/Core/ZB.MOM.WW.OtOpcUa.Cluster/ServiceCollectionExtensions.cs

@@ -241,7 +241,13 @@ public static class ServiceCollectionExtensions
 
         return new ClusterOptions
         {
-            SeedNodes = options.SeedNodes,
+            // When the bootstrap guard is on, DO NOT hand Akka the seed list: Akka.Cluster.Hosting
+            // auto-joins from ClusterOptions.SeedNodes the instant the ActorSystem starts, which is
+            // exactly the self-first race the guard exists to arbitrate. Left empty, the node starts
+            // unjoined and ClusterBootstrapCoordinator picks the order (founder self-first / joiner
+            // peer-first, after a reachability probe) and issues the single Cluster.JoinSeedNodes.
+            // The config seed list is still read — by the coordinator, off AkkaClusterOptions.SeedNodes.
+            SeedNodes = options.BootstrapGuard.Enabled ? Array.Empty<string>() : options.SeedNodes,
             Roles = options.Roles,
             SplitBrainResolver = IsKeepOldest(options) ? new KeepOldestOption { DownIfAlone = true } : null,
         };