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/Phase7Plan.cs

@@ -21,16 +21,21 @@ public sealed record Phase7Plan(
     IReadOnlyList<Phase7Plan.DriverDelta> ChangedDrivers,
     IReadOnlyList<ScriptedAlarmPlan> AddedAlarms,
     IReadOnlyList<ScriptedAlarmPlan> RemovedAlarms,
-    IReadOnlyList<Phase7Plan.AlarmDelta> ChangedAlarms)
+    IReadOnlyList<Phase7Plan.AlarmDelta> ChangedAlarms,
+    IReadOnlyList<GalaxyTagPlan> AddedGalaxyTags,
+    IReadOnlyList<GalaxyTagPlan> RemovedGalaxyTags,
+    IReadOnlyList<Phase7Plan.GalaxyTagDelta> ChangedGalaxyTags)
 {
     public bool IsEmpty =>
         AddedEquipment.Count == 0 && RemovedEquipment.Count == 0 && ChangedEquipment.Count == 0 &&
         AddedDrivers.Count == 0 && RemovedDrivers.Count == 0 && ChangedDrivers.Count == 0 &&
-        AddedAlarms.Count == 0 && RemovedAlarms.Count == 0 && ChangedAlarms.Count == 0;
+        AddedAlarms.Count == 0 && RemovedAlarms.Count == 0 && ChangedAlarms.Count == 0 &&
+        AddedGalaxyTags.Count == 0 && RemovedGalaxyTags.Count == 0 && ChangedGalaxyTags.Count == 0;
 
     public sealed record EquipmentDelta(EquipmentNode Previous, EquipmentNode Current);
     public sealed record DriverDelta(DriverInstancePlan Previous, DriverInstancePlan Current);
     public sealed record AlarmDelta(ScriptedAlarmPlan Previous, ScriptedAlarmPlan Current);
+    public sealed record GalaxyTagDelta(GalaxyTagPlan Previous, GalaxyTagPlan Current);
 }
 
 public static class Phase7Planner
@@ -61,10 +66,16 @@ public static class Phase7Planner
             a => a.ScriptedAlarmId,
             (a, b) => new Phase7Plan.AlarmDelta(a, b));
 
+        var (addedGalaxy, removedGalaxy, changedGalaxy) = DiffById(
+            previous.GalaxyTags, next.GalaxyTags,
+            t => t.TagId,
+            (a, b) => new Phase7Plan.GalaxyTagDelta(a, b));
+
         return new Phase7Plan(
             addedEq, removedEq, changedEq,
             addedDrv, removedDrv, changedDrv,
-            addedAlarm, removedAlarm, changedAlarm);
+            addedAlarm, removedAlarm, changedAlarm,
+            addedGalaxy, removedGalaxy, changedGalaxy);
     }
 
     private static (IReadOnlyList<T> Added, IReadOnlyList<T> Removed, IReadOnlyList<TDelta> Changed)