feat(cluster): manual central failover service — graceful Leave of the oldest Up member

Admin-triggered failover of the central pair. IManualFailoverService is declared in
CentralUI (plain strings, so that project stays Akka-free); AkkaManualFailoverService
implements it in the Host and is registered only in the Central branch.

- Leave, never Down: singletons hand over instead of being killed.
- Target = oldest Up member with the Central role, mirroring ActiveNodeEvaluator, so
  the node acted on is exactly the one hosting the singletons (never the leader,
  whose address-ordered definition diverges from singleton placement after a restart).
- Peer guard: returns null when fewer than 2 Up Central members — failing over a lone
  node is an outage, not a failover.
- Audited BEFORE the Leave is issued via ICentralAuditWriter: the acting node can be
  the one that goes away, and an audit written after could be lost to the shutdown it
  describes. Best-effort — audit failure never blocks the failover.

New audit taxonomy: AuditChannel.Cluster + AuditKind.ManualFailover (operator-initiated
topology actions are not script trust-boundary crossings, but are exactly what an audit
log exists to attribute). Lock-in tests updated 5->6 channels, 16->17 kinds.

alog.md §4 updated per the lock-in tests' contract. Both tables were already stale --
the Channel row omitted SecuredWrite and the Kind table claimed 10 while the code had
16 -- so they are completed here, not merely appended to.

ManualFailoverTests: 3 real-cluster tests (oldest leaves + survivor takes over, peer
guard refuses with a positive still-running assert, dry-run probe does not perturb).

tests/ZB.MOM.WW.ScadaBridge.Commons.Tests/Types/Enums/AuditEnumTests.cs

@@ -12,7 +12,7 @@ public class AuditEnumTests
     [Fact]
     public void AuditChannel_HasExactlyExpectedMembers()
     {
-        var expected = new[] { "ApiOutbound", "DbOutbound", "Notification", "ApiInbound", "SecuredWrite" };
+        var expected = new[] { "ApiOutbound", "DbOutbound", "Notification", "ApiInbound", "SecuredWrite", "Cluster" };
         var actual = Enum.GetValues(typeof(AuditChannel))
             .Cast<AuditChannel>()
             .Select(x => x.ToString())
@@ -23,7 +23,7 @@ public class AuditEnumTests
     }
 
     [Fact]
-    public void AuditKind_HasExactlySixteenExpectedMembers()
+    public void AuditKind_HasExactlySeventeenExpectedMembers()
     {
         var expected = new[]
         {
@@ -33,13 +33,14 @@ public class AuditEnumTests
             "SecuredWriteSubmit", "SecuredWriteApprove", "SecuredWriteReject", "SecuredWriteExecute",
             "SecuredWriteExpire",
             "ReconciliationAbandoned",
+            "ManualFailover",
         };
         var actual = Enum.GetValues(typeof(AuditKind))
             .Cast<AuditKind>()
             .Select(x => x.ToString())
             .ToArray();
 
-        Assert.Equal(16, actual.Length);
+        Assert.Equal(17, actual.Length);
         Assert.Equal(expected, actual);
     }