docs(mesh): fix stale redundancy-singleton comments + prove the driver fallback end-to-end

Review follow-ups on the redundancy re-home (Phase 6, Task 2):

- Program.cs: the hasDriver comment claimed IClusterRoleInfo "throws at host
  start if this driver node carries no cluster role" — false since the fallback
  landed. Rewrite it to describe the cluster-{ClusterId} scope with the driver-role
  fallback for legacy single-mesh / not-yet-migrated nodes.
- ClusterRoleInfo.cs: the SubscriberActor comment described RedundancyStateActor as
  the "admin-role singleton" — stale. Note it is now a cluster-{ClusterId}-scoped
  singleton spawned on every driver node (LeaderChanged stays a no-op here).
- Add a host/registry-level test for the driver-role FALLBACK path (roles ["driver"],
  no cluster role) closing the coverage asymmetry — it was proven only in the pure
  helper. Asserts the node boots and registers RedundancyStateActorKey, i.e. the boot
  the earlier throw-based version would have aborted.

The boot helper now supplies an in-memory IDbContextFactory and a fake IClusterRoleInfo:
Akka.Hosting invokes singleton props factories eagerly at StartAsync, and the admin
ClusterNodeAddressReconciler factory reads both (the latter for its per-cluster reconcile
scope, added by a sibling Phase 6 task) — without them the admin boot NREs.

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

tests/Server/ZB.MOM.WW.OtOpcUa.ControlPlane.Tests/RedundancyStateSingletonRehomeTests.cs

@@ -1,6 +1,7 @@
 using Akka.Actor;
 using Akka.Cluster.Hosting;
 using Akka.Hosting;
+using Microsoft.EntityFrameworkCore;
 using Microsoft.Extensions.DependencyInjection;
 using Microsoft.Extensions.Hosting;
 using Microsoft.Extensions.Options;
@@ -9,6 +10,7 @@ using Xunit;
 using ZB.MOM.WW.OtOpcUa.Cluster;
 using ZB.MOM.WW.OtOpcUa.Commons.Interfaces;
 using ZB.MOM.WW.OtOpcUa.Commons.Types;
+using ZB.MOM.WW.OtOpcUa.Configuration;
 
 namespace ZB.MOM.WW.OtOpcUa.ControlPlane.Tests;
 
@@ -87,6 +89,25 @@ public sealed class RedundancyStateSingletonRehomeTests
             "every driver node hosts its own mesh's redundancy singleton (Phase 6)");
     }
 
+    /// <summary>
+    /// The driver-role FALLBACK end-to-end: a node carrying only <c>driver</c> (no cluster role — a
+    /// legacy single-mesh node or the <c>TwoNodeClusterHarness</c>) still boots and registers the
+    /// redundancy singleton via the new extension. Proves the fallback works at the host/registry
+    /// level, not just in the pure helper — the boot that the earlier throw-based version would have
+    /// aborted.
+    /// </summary>
+    [Fact]
+    public async Task Driver_role_fallback_registers_the_singleton_end_to_end()
+    {
+        var registry = await BootAndGetRegistryAsync(
+            roles: new[] { "driver" },
+            configure: ab => ab.WithOtOpcUaClusterRedundancySingleton(
+                new FakeClusterRoleInfo(clusterRole: null, clusterId: null)));
+
+        registry.TryGet<RedundancyStateActorKey>(out _).ShouldBeTrue(
+            "a driver node with no cluster role falls back to the driver role and still boots + registers");
+    }
+
     /// <summary>
     /// Boots a real host through the production cluster bootstrap (Port=0, no seeds, so it never forms
     /// a cluster and no singleton props factory runs — only the proxies register), applies the
@@ -112,6 +133,15 @@ public sealed class RedundancyStateSingletonRehomeTests
             services.AddSingleton<IOptions<AkkaClusterOptions>>(Options.Create(options));
             // The admin WithActors block reads IOptions<MeshTransportOptions>.Value at spawn time.
             services.AddSingleton<IOptions<MeshTransportOptions>>(Options.Create(new MeshTransportOptions()));
+            // Akka.Hosting invokes every singleton's props factory eagerly at StartAsync; the admin
+            // singletons (coordinator/audit/reconciler) read IDbContextFactory<OtOpcUaConfigDbContext>,
+            // so supply an in-memory one or those factories NRE. The driver-only boots don't touch it.
+            services.AddDbContextFactory<OtOpcUaConfigDbContext>(
+                o => o.UseInMemoryDatabase($"rehome-test-{Guid.NewGuid():N}"));
+            // The admin ClusterNodeAddressReconciler props factory resolves IClusterRoleInfo (for its
+            // per-cluster reconcile scope). A null cluster role ⇒ full-fleet reconcile, which is all
+            // this test needs — it never asserts on the reconciler, only on the singleton registry.
+            services.AddSingleton<IClusterRoleInfo>(new FakeClusterRoleInfo(clusterRole: null, clusterId: null));
             services.AddAkka("otopcua-rehome-test", (ab, sp) =>
             {
                 ab.WithOtOpcUaClusterBootstrap(sp);