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).

docker-dev/seed/seed-clusters.sql

@@ -150,6 +150,36 @@ IF NOT EXISTS (SELECT 1 FROM dbo.DriverInstance WHERE DriverInstanceId = 'MAIN-g
   }
 }');
 
+------------------------------------------------------------------------------
+-- Galaxy test tags — TestMachine_001.TestAlarm001..003
+--
+-- SystemPlatform-namespace tags have EquipmentId=NULL and use FolderPath +
+-- Name to address the MXAccess item. The Galaxy driver subscribes via the
+-- "FolderPath.Name" MXAccess reference form; OPC UA browse path is the
+-- equivalent "FolderPath/Name" under the SystemPlatform namespace.
+------------------------------------------------------------------------------
+
+IF NOT EXISTS (SELECT 1 FROM dbo.Tag WHERE TagId = 'MAIN-galaxy-TestMachine_001-TestAlarm001')
+    INSERT INTO dbo.Tag
+        (TagRowId, TagId, DriverInstanceId, DeviceId, EquipmentId, Name, FolderPath, DataType, AccessLevel, WriteIdempotent, PollGroupId, TagConfig)
+    VALUES
+        (NEWID(), 'MAIN-galaxy-TestMachine_001-TestAlarm001', 'MAIN-galaxy-mxgw', NULL, NULL,
+         'TestAlarm001', 'TestMachine_001', 'Boolean', 0, 0, NULL, N'{}');
+
+IF NOT EXISTS (SELECT 1 FROM dbo.Tag WHERE TagId = 'MAIN-galaxy-TestMachine_001-TestAlarm002')
+    INSERT INTO dbo.Tag
+        (TagRowId, TagId, DriverInstanceId, DeviceId, EquipmentId, Name, FolderPath, DataType, AccessLevel, WriteIdempotent, PollGroupId, TagConfig)
+    VALUES
+        (NEWID(), 'MAIN-galaxy-TestMachine_001-TestAlarm002', 'MAIN-galaxy-mxgw', NULL, NULL,
+         'TestAlarm002', 'TestMachine_001', 'Boolean', 0, 0, NULL, N'{}');
+
+IF NOT EXISTS (SELECT 1 FROM dbo.Tag WHERE TagId = 'MAIN-galaxy-TestMachine_001-TestAlarm003')
+    INSERT INTO dbo.Tag
+        (TagRowId, TagId, DriverInstanceId, DeviceId, EquipmentId, Name, FolderPath, DataType, AccessLevel, WriteIdempotent, PollGroupId, TagConfig)
+    VALUES
+        (NEWID(), 'MAIN-galaxy-TestMachine_001-TestAlarm003', 'MAIN-galaxy-mxgw', NULL, NULL,
+         'TestAlarm003', 'TestMachine_001', 'Boolean', 0, 0, NULL, N'{}');
+
 COMMIT TRANSACTION;
 
 ------------------------------------------------------------------------------
@@ -162,3 +192,4 @@ SELECT NodeId, ClusterId, Host, OpcUaPort, ApplicationUri, ServiceLevelBase
 SELECT NamespaceId, ClusterId, Kind, NamespaceUri FROM dbo.Namespace ORDER BY ClusterId, NamespaceId;
 SELECT DriverInstanceId, ClusterId, DriverType, NamespaceId, Name
   FROM dbo.DriverInstance ORDER BY ClusterId, DriverInstanceId;
+SELECT TagId, DriverInstanceId, FolderPath, Name, DataType FROM dbo.Tag ORDER BY DriverInstanceId, FolderPath, Name;