docs(archreview #11): correct Critical 1 premise — SBR was already active on master, not NoDowning

Akka.Cluster.Hosting's WithClustering enables an SBR downing provider by default
(applies SplitBrainResolverOption.Default when ClusterOptions.SplitBrainResolver is
null), which reads the pre-existing akka.conf keep-oldest block. So the cluster was
NOT running NoDowning before Critical 1 and hard-crash failover already worked — the
typed KeepOldestOption is reinforcing/explicit-in-code, not the sole activator.

Corrects the inaccurate 'HOCON inert / NoDowning / never fails over' framing in:
- ServiceCollectionExtensions.BuildClusterOptions XML comment
- akka.conf split-brain-resolver comment
- docs/Redundancy.md Split-brain section
- SplitBrainResolverActivationTests summary + assertion message (+ method rename)

No code revert (the typed option is correct belt-and-suspenders). Cluster.Tests 29/29.
Surfaced by the #9 hard-kill failover negative control.

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

@@ -84,16 +84,19 @@ public static class ServiceCollectionExtensions
     }
 
     /// <summary>
-    /// Builds the <see cref="ClusterOptions"/> for the fused-host cluster, including the
-    /// <b>activating</b> split-brain-resolver registration.
+    /// Builds the <see cref="ClusterOptions"/> for the fused-host cluster, setting the split-brain-resolver
+    /// strategy <b>explicitly in code</b>.
     ///
-    /// The <c>split-brain-resolver</c> HOCON block in <c>Resources/akka.conf</c> is inert on its own —
-    /// Akka.NET only runs the resolver when a downing provider is registered, otherwise the cluster
-    /// falls back to <b>NoDowning</b> and a hard-crashed node is never downed (singletons and the
-    /// <c>driver</c> role-leader never fail over, and a partition leaves both redundancy sides at
-    /// ServiceLevel 240 forever). Setting <see cref="ClusterOptions.SplitBrainResolver"/> is what
-    /// activates <c>Akka.Cluster.SBR.SplitBrainResolverProvider</c> under the hood — this is the piece
-    /// that was missing (arch-review 03/S1).
+    /// <b>Activation note (arch-review 03/S1, corrected):</b> Akka.Cluster.Hosting's <c>WithClustering</c>
+    /// already enables an SBR downing provider <b>by default</b> — when <see cref="ClusterOptions.SplitBrainResolver"/>
+    /// is <c>null</c> it applies <c>SplitBrainResolverOption.Default</c>, which registers
+    /// <c>Akka.Cluster.SBR.SplitBrainResolverProvider</c> and reads the <c>split-brain-resolver</c> HOCON block in
+    /// <c>Resources/akka.conf</c>. So the cluster was <b>not</b> running <c>NoDowning</c> before this option was
+    /// set: the pre-existing akka.conf <c>keep-oldest</c> block was already active, and hard-crashed nodes already
+    /// failed over. Setting this typed option makes the strategy <b>explicit in code</b> (independent of the
+    /// framework default) rather than being the sole activator — it is reinforcing/belt-and-suspenders and
+    /// produces the same effective behavior. (Only an <i>explicit</i> <c>NoDowning</c>, e.g.
+    /// <c>downing-provider-class = ""</c>, would disable failover.)
     ///
     /// <see cref="KeepOldestOption"/> with <c>DownIfAlone=true</c> mirrors the HOCON intent and is the
     /// correct strategy for a 2-node warm-redundancy pair: on an even split the oldest member (typically
@@ -103,7 +106,7 @@ public static class ServiceCollectionExtensions
     /// typed option cannot express it (it must stay ≥ <c>failure-detector.acceptable-heartbeat-pause</c>).
     /// </summary>
     /// <param name="options">The bound cluster options carrying seed nodes and roles.</param>
-    /// <returns>The cluster options with seed nodes, roles, and the activated split-brain resolver.</returns>
+    /// <returns>The cluster options with seed nodes, roles, and the explicit split-brain-resolver strategy.</returns>
     public static ClusterOptions BuildClusterOptions(AkkaClusterOptions options)
     {
         return new ClusterOptions