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

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

@@ -211,7 +211,9 @@ public sealed class ClusterRoleInfo : IClusterRoleInfo, IDisposable
             Receive<ClusterEvent.IMemberEvent>(e => owner.HandleMemberEvent(e));
             Receive<ClusterEvent.RoleLeaderChanged>(e => owner.HandleRoleLeaderChanged(e));
             // LeaderChanged is intentionally a no-op here: ServiceLevel lives in RedundancyStateActor
-            // (admin-role singleton) and has no consumer on this ClusterRoleInfo path by design.
+            // (per-cluster mesh Phase 6: a cluster-{ClusterId}-scoped singleton spawned on every
+            // driver node, no longer the admin-role singleton it once was) and has no consumer on this
+            // ClusterRoleInfo path by design.
             Receive<ClusterEvent.LeaderChanged>(_ => { });
             Receive<ClusterEvent.CurrentClusterState>(_ => { /* seeded from initial snapshot */ });
         }