feat(opcua): #85 UNS Area/Line/Equipment folder hierarchy in SDK

Phase7Composer now carries UnsAreaProjection + UnsLineProjection lists so
the applier can materialise the full UNS topology in the OPC UA address
space. New IOpcUaAddressSpaceSink.EnsureFolder(folderNodeId, parentNodeId,
displayName) seam (no-op default, recorded in tests, forwarded by
DeferredAddressSpaceSink, implemented by SdkAddressSpaceSink). The SDK-
side OtOpcUaNodeManager gains an EnsureFolder API that creates
FolderState nodes with proper parent linkage; RebuildAddressSpace now
clears folders too so re-applies don't accumulate stale topology.

Phase7Applier.MaterialiseHierarchy walks composition.UnsAreas →
composition.UnsLines → composition.EquipmentNodes, calling EnsureFolder
with the correct parent at each level. Idempotent — calling twice with
the same composition is a no-op. OpcUaPublishActor.HandleRebuild invokes
it after Phase7Applier.Apply so OPC UA clients browsing the server now
see Area/Line/Equipment as proper folders rather than flat tag ids.

DeploymentArtifact.ParseComposition reads UnsAreas + UnsLines from the
JSON snapshot the ControlPlane emits, populating the new fields when
present.

Phase7Composer.Compose now accepts UnsAreas + UnsLines; a 3-arg overload
preserves the old signature for legacy callers + existing tests. The
Phase7CompositionResult convenience ctor likewise keeps the planner
tests working without UNS data.

3 new hierarchy tests (pure unit + boot-verify against a real
OtOpcUaSdkServer); OpcUaServer suite is 48/48 green (was 45, +3),
Runtime 74/74 unchanged.

Closes #85.

src/Server/ZB.MOM.WW.OtOpcUa.OpcUaServer/OtOpcUaNodeManager.cs

@@ -27,6 +27,7 @@ public sealed class OtOpcUaNodeManager : CustomNodeManager2
     public const string DefaultNamespaceUri = "https://zb.com/otopcua/ns";
 
     private readonly ConcurrentDictionary<string, BaseDataVariableState> _variables = new(StringComparer.Ordinal);
+    private readonly ConcurrentDictionary<string, FolderState> _folders = new(StringComparer.Ordinal);
     private FolderState? _root;
 
     public OtOpcUaNodeManager(IServerInternal server, ApplicationConfiguration configuration)
@@ -36,6 +37,7 @@ public sealed class OtOpcUaNodeManager : CustomNodeManager2
     }
 
     public int VariableCount => _variables.Count;
+    public int FolderCount => _folders.Count;
 
     /// <summary>
     ///     Apply a value write from <see cref="IOpcUaAddressSpaceSink.WriteValue"/>. Creates the
@@ -73,9 +75,43 @@ public sealed class OtOpcUaNodeManager : CustomNodeManager2
         }
     }
 
-    /// <summary>Clear every registered variable from the address space. Phase7Applier calls this
-    /// when Equipment/Alarm topology changes; the populator then re-adds via WriteValue on the
-    /// next pass.</summary>
+    /// <summary>
+    ///     Ensure a folder node exists at <paramref name="folderNodeId"/> with the given display
+    ///     name, parented under <paramref name="parentNodeId"/> (or the namespace root when null).
+    ///     #85 — used by <see cref="Phase7Applier"/> to materialise the UNS Area/Line/Equipment
+    ///     folder hierarchy. Idempotent: the second call with the same id returns the cached
+    ///     folder so adding child variables under it still works.
+    /// </summary>
+    public void EnsureFolder(string folderNodeId, string? parentNodeId, string displayName)
+    {
+        ArgumentException.ThrowIfNullOrEmpty(folderNodeId);
+        ArgumentException.ThrowIfNullOrEmpty(displayName);
+
+        if (_folders.ContainsKey(folderNodeId)) return;
+
+        lock (Lock)
+        {
+            if (_folders.ContainsKey(folderNodeId)) return;
+
+            var parent = ResolveParentFolder(parentNodeId);
+            var folder = new FolderState(parent)
+            {
+                NodeId = new NodeId(folderNodeId, NamespaceIndex),
+                BrowseName = new QualifiedName(folderNodeId, NamespaceIndex),
+                DisplayName = displayName,
+                EventNotifier = EventNotifiers.None,
+                TypeDefinitionId = ObjectTypeIds.FolderType,
+                ReferenceTypeId = ReferenceTypeIds.Organizes,
+            };
+            parent.AddChild(folder);
+            AddPredefinedNode(SystemContext, folder);
+            _folders[folderNodeId] = folder;
+        }
+    }
+
+    /// <summary>Clear every registered variable + folder from the address space. Phase7Applier
+    /// calls this when Equipment/Alarm topology changes; the populator then re-adds via
+    /// EnsureFolder + WriteValue on the next pass.</summary>
     public void RebuildAddressSpace()
     {
         lock (Lock)
@@ -86,9 +122,22 @@ public sealed class OtOpcUaNodeManager : CustomNodeManager2
                 PredefinedNodes?.Remove(v.NodeId);
             }
             _variables.Clear();
+
+            foreach (var f in _folders.Values)
+            {
+                f.Parent?.RemoveChild(f);
+                PredefinedNodes?.Remove(f.NodeId);
+            }
+            _folders.Clear();
         }
     }
 
+    private FolderState ResolveParentFolder(string? parentNodeId)
+    {
+        if (string.IsNullOrEmpty(parentNodeId)) return _root!;
+        return _folders.TryGetValue(parentNodeId, out var existing) ? existing : _root!;
+    }
+
     /// <inheritdoc />
     public override void CreateAddressSpace(IDictionary<NodeId, IList<IReference>> externalReferences)
     {