review(Cluster): record findings + fix snapshot consistency, dispose, stale docs

Code review at HEAD 7286d320. Cluster-001 (SeedFromCurrentState reads from one
snapshot), Cluster-003 (HoconLoader double-dispose), Cluster-004 (stale akka.conf
header), Cluster-005 (ServiceLevelCalculator tests added to Cluster.Tests). Cluster-002
deferred (no production caller).

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

@@ -99,7 +99,7 @@ public sealed class ClusterRoleInfo : IClusterRoleInfo, IDisposable
 
             foreach (var role in snapshot.Members.SelectMany(m => m.Roles).Distinct())
             {
-                var leaderAddr = _cluster.State.RoleLeader(role);
+                var leaderAddr = snapshot.RoleLeader(role);
                 _roleLeaders[role] = leaderAddr is not null
                     ? snapshot.Members.FirstOrDefault(m => m.Address == leaderAddr)
                     : null;