feat(opcua): add EquipmentVirtualTagPlan to Phase7 composition

Adds the EquipmentVirtualTagPlan sealed record (VirtualTagId, EquipmentId,
FolderPath, Name, DataType, Expression, DependencyRefs) and the
EquipmentVirtualTags init-only member on Phase7CompositionResult, mirroring
the existing EquipmentTagPlan / EquipmentTags pattern. Type-only: no producer
logic yet. Two new tests cover the default-empty guarantee and the record shape.

src/Server/ZB.MOM.WW.OtOpcUa.OpcUaServer/Phase7Composer.cs

@@ -56,6 +56,10 @@ public sealed record Phase7CompositionResult(
     ///     constructor + call site keeps compiling unchanged; new producers set it via initializer.
     /// </summary>
     public IReadOnlyList<EquipmentTagPlan> EquipmentTags { get; init; } = Array.Empty<EquipmentTagPlan>();
+
+    /// <summary>Equipment-namespace VirtualTags. See <see cref="EquipmentVirtualTagPlan"/>. Init-only,
+    /// defaults empty so every existing constructor + call site keeps compiling.</summary>
+    public IReadOnlyList<EquipmentVirtualTagPlan> EquipmentVirtualTags { get; init; } = Array.Empty<EquipmentVirtualTagPlan>();
 }
 
 public sealed record UnsAreaProjection(string UnsAreaId, string DisplayName);
@@ -100,6 +104,26 @@ public sealed record EquipmentTagPlan(
     string DataType,
     string FullName);
 
+/// <summary>
+///     One Equipment-namespace VirtualTag from a <see cref="VirtualTag"/> row (joined to its
+///     <see cref="Script"/> for the expression). The VirtualTag value analogue of
+///     <see cref="EquipmentTagPlan"/>: <c>Phase7Applier.MaterialiseEquipmentVirtualTags</c>
+///     materialises each as a Variable under its equipment folder with a folder-scoped NodeId
+///     (<c>EquipmentId/Name</c>, or <c>EquipmentId/FolderPath/Name</c> when a sub-folder is set),
+///     and <c>VirtualTagHostActor</c> spawns a <c>VirtualTagActor</c> per plan that evaluates
+///     <see cref="Expression"/> over <see cref="DependencyRefs"/> and publishes the value back to
+///     that NodeId. <see cref="DependencyRefs"/> = the distinct <c>ctx.GetTag("…")</c> literals in
+///     the script source.
+/// </summary>
+public sealed record EquipmentVirtualTagPlan(
+    string VirtualTagId,
+    string EquipmentId,
+    string FolderPath,
+    string Name,
+    string DataType,
+    string Expression,
+    IReadOnlyList<string> DependencyRefs);
+
 /// <summary>
 /// Pure composer that flattens the live-edit DB tables into the address-space build plan a
 /// driver-role host needs. Same inputs → same outputs, no logging, no DB writes. The driver-role