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/Core/ZB.MOM.WW.OtOpcUa.Commons/OpcUa/IOpcUaAddressSpaceSink.cs

@@ -22,6 +22,16 @@ public interface IOpcUaAddressSpaceSink
     /// </summary>
     void EnsureFolder(string folderNodeId, string? parentNodeId, string displayName);
 
+    /// <summary>
+    ///     Ensure a Variable node exists at <paramref name="variableNodeId"/>, parented under
+    ///     <paramref name="parentFolderNodeId"/> (or the namespace root when null). Created with
+    ///     Bad quality + null value; subsequent <see cref="WriteValue"/> calls update both.
+    ///     Used by <c>Phase7Applier</c> to materialise Galaxy / SystemPlatform tags ahead of any
+    ///     driver-side subscribe so OPC UA clients can browse them. Idempotent.
+    /// </summary>
+    /// <param name="dataType">OPC UA built-in type name ("Boolean" / "Int32" / "Float" / etc.).</param>
+    void EnsureVariable(string variableNodeId, string? parentFolderNodeId, string displayName, string dataType);
+
     /// <summary>
     ///     Tear down + repopulate the address space. Called by <c>OpcUaPublishActor</c> after a
     ///     successful deployment apply so the node manager reflects the new config. Idempotent.
@@ -42,5 +52,6 @@ public sealed class NullOpcUaAddressSpaceSink : IOpcUaAddressSpaceSink
     public void WriteValue(string nodeId, object? value, OpcUaQuality quality, DateTime sourceTimestampUtc) { }
     public void WriteAlarmState(string alarmNodeId, bool active, bool acknowledged, DateTime sourceTimestampUtc) { }
     public void EnsureFolder(string folderNodeId, string? parentNodeId, string displayName) { }
+    public void EnsureVariable(string variableNodeId, string? parentFolderNodeId, string displayName, string dataType) { }
     public void RebuildAddressSpace() { }
 }