fix(opcua): structural equality for EquipmentVirtualTagPlan so no-op redeploys diff empty

IReadOnlyList<string> DependencyRefs compared by reference in the auto-generated record
equality, causing every VirtualTag with dependencies to be flagged "Changed" on every parse
(fresh list instances from composer and artifact-decoder). Add Equals/GetHashCode overrides
with element-wise ordinal comparison so Phase7Plan.IsEmpty short-circuits a no-op redeploy.
Add regression test Identical_virtualtag_snapshots_diff_to_empty_plan (separate list instances,
same contents → IsEmpty true). Add TODO comment in Phase7Applier near needsRebuild predicate.

src/Server/ZB.MOM.WW.OtOpcUa.OpcUaServer/Phase7Applier.cs

@@ -81,6 +81,7 @@ public sealed class Phase7Applier
         // VirtualTag topology requires a real address-space rebuild. Driver-instance changes don't
         // touch the address-space topology directly — they go through DriverHostActor's spawn-plan
         // in Runtime.
+        // TODO(equipment-virtualtags): when MaterialiseEquipmentVirtualTags drives per-delta sink work, revisit whether ChangedEquipmentVirtualTags should also force needsRebuild.
         var needsRebuild =
             plan.AddedEquipment.Count > 0 || plan.RemovedEquipment.Count > 0 ||
             plan.AddedAlarms.Count > 0 || plan.RemovedAlarms.Count > 0 ||

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

@@ -122,7 +122,31 @@ public sealed record EquipmentVirtualTagPlan(
     string Name,
     string DataType,
     string Expression,
-    IReadOnlyList<string> DependencyRefs);
+    IReadOnlyList<string> DependencyRefs)
+{
+    /// <summary>Structural equality: the auto-generated record equality would compare
+    /// <see cref="DependencyRefs"/> (an interface-typed list) BY REFERENCE, flagging every
+    /// VirtualTag as "changed" on every parse (fresh list instances). Compare it element-wise
+    /// so a no-op redeploy diffs empty.</summary>
+    public bool Equals(EquipmentVirtualTagPlan? other) =>
+        other is not null &&
+        VirtualTagId == other.VirtualTagId &&
+        EquipmentId == other.EquipmentId &&
+        FolderPath == other.FolderPath &&
+        Name == other.Name &&
+        DataType == other.DataType &&
+        Expression == other.Expression &&
+        DependencyRefs.SequenceEqual(other.DependencyRefs, StringComparer.Ordinal);
+
+    public override int GetHashCode()
+    {
+        var hash = new HashCode();
+        hash.Add(VirtualTagId); hash.Add(EquipmentId); hash.Add(FolderPath);
+        hash.Add(Name); hash.Add(DataType); hash.Add(Expression);
+        foreach (var r in DependencyRefs) hash.Add(r, StringComparer.Ordinal);
+        return hash.ToHashCode();
+    }
+}
 
 /// <summary>
 /// Pure composer that flattens the live-edit DB tables into the address-space build plan a

tests/Server/ZB.MOM.WW.OtOpcUa.OpcUaServer.Tests/Phase7PlannerTests.cs

@@ -139,6 +139,44 @@ public sealed class Phase7PlannerTests
         plan.RemovedEquipmentVirtualTags.ShouldBeEmpty();
     }
 
+    /// <summary>Regression guard for structural equality on <see cref="EquipmentVirtualTagPlan.DependencyRefs"/>:
+    /// two snapshots containing the SAME VirtualTag built from SEPARATE list instances must diff to an empty plan
+    /// (IReadOnlyList equality is BY REFERENCE without the custom Equals override, so every VirtualTag with
+    /// dependencies would be wrongly flagged "Changed" on every parse, preventing IsEmpty short-circuits).</summary>
+    [Fact]
+    public void Identical_virtualtag_snapshots_diff_to_empty_plan()
+    {
+        // Two separate list instances with identical contents — proves structural (not reference) equality.
+        var refsA = new[] { "EQ1.Speed", "EQ1.Torque" };
+        var refsB = new[] { "EQ1.Speed", "EQ1.Torque" };
+
+        var prev = new Phase7CompositionResult(
+            Array.Empty<EquipmentNode>(), Array.Empty<DriverInstancePlan>(), Array.Empty<ScriptedAlarmPlan>())
+        {
+            EquipmentVirtualTags = new[]
+            {
+                new EquipmentVirtualTagPlan("vt-1", "eq-1", FolderPath: "", Name: "Efficiency", DataType: "Float",
+                    Expression: "ctx.GetTag(\"EQ1.Speed\") / ctx.GetTag(\"EQ1.Torque\")", DependencyRefs: refsA),
+            },
+        };
+        var next = new Phase7CompositionResult(
+            Array.Empty<EquipmentNode>(), Array.Empty<DriverInstancePlan>(), Array.Empty<ScriptedAlarmPlan>())
+        {
+            EquipmentVirtualTags = new[]
+            {
+                new EquipmentVirtualTagPlan("vt-1", "eq-1", FolderPath: "", Name: "Efficiency", DataType: "Float",
+                    Expression: "ctx.GetTag(\"EQ1.Speed\") / ctx.GetTag(\"EQ1.Torque\")", DependencyRefs: refsB),
+            },
+        };
+
+        var plan = Phase7Planner.Compute(prev, next);
+
+        plan.IsEmpty.ShouldBeTrue();
+        plan.ChangedEquipmentVirtualTags.ShouldBeEmpty();
+        plan.AddedEquipmentVirtualTags.ShouldBeEmpty();
+        plan.RemovedEquipmentVirtualTags.ShouldBeEmpty();
+    }
+
     /// <summary>Verifies that new equipment goes to the AddedEquipment list.</summary>
     [Fact]
     public void New_equipment_goes_to_AddedEquipment()