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/Resources/akka.conf

@@ -37,12 +37,13 @@ akka {
         roles = []
         min-nr-of-members = 1
 
-        # Split-brain resolver. This HOCON block is the TUNING source only — it does NOT
-        # activate the resolver. Activation happens in code via the typed
-        # ClusterOptions.SplitBrainResolver (KeepOldestOption { DownIfAlone = true }) set in
-        # ServiceCollectionExtensions.BuildClusterOptions; without that the cluster runs Akka's
-        # default NoDowning and hard-crashed nodes never fail over (arch-review 03/S1).
-        # active-strategy + keep-oldest.down-if-alone below MUST match the typed option.
+        # Split-brain resolver (arch-review 03/S1). This block IS active: Akka.Cluster.Hosting's
+        # WithClustering enables an SBR downing provider by default (it applies
+        # SplitBrainResolverOption.Default when ClusterOptions.SplitBrainResolver is null), and that
+        # provider reads this block. ServiceCollectionExtensions.BuildClusterOptions additionally sets
+        # the typed KeepOldestOption { DownIfAlone = true } to make the strategy EXPLICIT in code —
+        # its active-strategy + keep-oldest.down-if-alone MUST match this block. (The cluster is NOT
+        # NoDowning; only an explicit downing-provider-class = "" would disable failover.)
         # stable-after must stay >= failure-detector.acceptable-heartbeat-pause (10s) + margin.
         split-brain-resolver {
             active-strategy = "keep-oldest"