feat(opcua): materialise SystemPlatform tags (Galaxy) as OPC UA variables

Closes the gap where Tag rows with EquipmentId=NULL + Namespace.Kind=SystemPlatform
(Galaxy hierarchy) existed in ConfigDb but were never surfaced in the OPC UA
address space. Now they materialise as Variable nodes under a folder named for
their FolderPath, browseable through any OPC UA client.

Layers touched:

- IOpcUaAddressSpaceSink: new EnsureVariable(nodeId, parentFolderId, displayName,
  dataType) signature on the sink interface, NullSink, DeferredSink, SdkSink.
- OtOpcUaNodeManager.EnsureVariable: creates a BaseDataVariableState parented
  under the named folder (or root), initial Value=null +
  StatusCode=BadWaitingForInitialData; resolves Tag.DataType strings to the
  matching OPC UA built-in NodeId. Idempotent.
- Phase7CompositionResult: new GalaxyTags collection of GalaxyTagPlan records
  carrying (TagId, DriverInstanceId, FolderPath, DisplayName, DataType,
  MxAccessRef). Constructor overloads keep existing call sites compiling.
- Phase7Composer.Compose: now takes Tag + Namespace inputs, filters for
  SystemPlatform-namespace tags with EquipmentId=NULL, emits GalaxyTagPlan
  rows with MXAccess ref "FolderPath.Name".
- Phase7Plan: new AddedGalaxyTags / RemovedGalaxyTags / ChangedGalaxyTags
  collections + GalaxyTagDelta record; IsEmpty + needsRebuild updated.
- Phase7Planner.Compute: diffs GalaxyTags by TagId via existing DiffById helper.
- DeploymentArtifact.ParseComposition: reads the Tags + Namespaces +
  DriverInstances arrays the ConfigComposer already emits, applies the same
  SystemPlatform filter, returns the same GalaxyTagPlan list as the composer
  so artifact-side and compose-side plans agree.
- Phase7Applier: new MaterialiseGalaxyTags pass that ensures one folder per
  distinct FolderPath then one Variable per tag. NodeId for the variable is
  "<FolderPath>.<Name>" matching the MXAccess ref so the future Galaxy
  SubscribeBulk wiring can address them directly.
- OpcUaPublishActor.RebuildAddressSpace: invokes MaterialiseGalaxyTags after
  MaterialiseHierarchy. _lastApplied initialiser updated for the new ctor.
- seed-clusters.sql: pre-existing TestMachine_001.TestAlarm001..003 rows
  needed no change — the composer/applier now picks them up automatically.

Verified end-to-end via docker-dev: deploy click → driver-a logs
"Phase7Applier: Galaxy tags materialised (tags=3, folders=1)" → OPC UA Client
CLI browses the three Variable nodes under TestMachine_001 folder. Reads
return BadWaitingForInitialData status (expected — Galaxy driver's
SubscribeBulk wiring to push values into the nodes is the remaining
follow-up).

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

@@ -109,6 +109,69 @@ public sealed class OtOpcUaNodeManager : CustomNodeManager2
         }
     }
 
+    /// <summary>
+    ///     Ensure a Variable node exists at <paramref name="variableNodeId"/> parented under
+    ///     <paramref name="parentFolderNodeId"/> (or root when null). Initial value=null, quality=Bad,
+    ///     timestamp=epoch — <see cref="WriteValue"/> fills these in once driver data flows.
+    ///     Idempotent. Materialises Galaxy / SystemPlatform tags so they're browseable before the
+    ///     Galaxy driver issues SubscribeBulk.
+    /// </summary>
+    public void EnsureVariable(string variableNodeId, string? parentFolderNodeId, string displayName, string dataType)
+    {
+        ArgumentException.ThrowIfNullOrEmpty(variableNodeId);
+        ArgumentException.ThrowIfNullOrEmpty(displayName);
+
+        // If already present, leave it alone (idempotent re-applies).
+        if (_variables.ContainsKey(variableNodeId)) return;
+
+        lock (Lock)
+        {
+            if (_variables.ContainsKey(variableNodeId)) return;
+
+            var parent = ResolveParentFolder(parentFolderNodeId);
+            var variable = new BaseDataVariableState(parent)
+            {
+                NodeId = new NodeId(variableNodeId, NamespaceIndex),
+                BrowseName = new QualifiedName(variableNodeId, NamespaceIndex),
+                DisplayName = displayName,
+                TypeDefinitionId = VariableTypeIds.BaseDataVariableType,
+                ReferenceTypeId = ReferenceTypeIds.Organizes,
+                DataType = ResolveBuiltInDataType(dataType),
+                ValueRank = ValueRanks.Scalar,
+                AccessLevel = AccessLevels.CurrentRead,
+                UserAccessLevel = AccessLevels.CurrentRead,
+                Historizing = false,
+                Value = null,
+                StatusCode = StatusCodes.BadWaitingForInitialData,
+                Timestamp = DateTime.MinValue,
+            };
+            parent.AddChild(variable);
+            AddPredefinedNode(SystemContext, variable);
+            _variables[variableNodeId] = variable;
+        }
+    }
+
+    /// <summary>Map a Tag.DataType string ("Boolean", "Int32", "Float", "Double", "String",
+    /// "DateTime") to the OPC UA built-in NodeId. Unknown names fall back to BaseDataType
+    /// (matches CreateVariable's default for lazy-created nodes).</summary>
+    private static NodeId ResolveBuiltInDataType(string dataType) => dataType switch
+    {
+        "Boolean" => DataTypeIds.Boolean,
+        "SByte" => DataTypeIds.SByte,
+        "Byte" => DataTypeIds.Byte,
+        "Int16" => DataTypeIds.Int16,
+        "UInt16" => DataTypeIds.UInt16,
+        "Int32" => DataTypeIds.Int32,
+        "UInt32" => DataTypeIds.UInt32,
+        "Int64" => DataTypeIds.Int64,
+        "UInt64" => DataTypeIds.UInt64,
+        "Float" => DataTypeIds.Float,
+        "Double" => DataTypeIds.Double,
+        "String" => DataTypeIds.String,
+        "DateTime" => DataTypeIds.DateTime,
+        _ => DataTypeIds.BaseDataType,
+    };
+
     /// <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>