refactor(config): materialise collision groups once; note VirtualTag folder coupling

src/Core/ZB.MOM.WW.OtOpcUa.Configuration/Validation/DraftValidator.cs

@@ -47,14 +47,17 @@ public static class DraftValidator
         var signals = draft.Tags
             .Where(t => t.EquipmentId is not null)
             .Select(t => (Key: Key(t.EquipmentId!, t.FolderPath, t.Name), Eq: t.EquipmentId!, t.Name))
+            // VirtualTag has no FolderPath column today — null is correct here; update if it ever gains one.
             .Concat(draft.VirtualTags
                 .Select(v => (Key: Key(v.EquipmentId, null, v.Name), Eq: v.EquipmentId, v.Name)));
 
-        foreach (var g in signals.GroupBy(s => s.Key, StringComparer.Ordinal).Where(g => g.Count() > 1))
+        foreach (var g in signals.GroupBy(s => s.Key, StringComparer.Ordinal))
         {
-            var f = g.First();
+            var items = g.ToList();
+            if (items.Count <= 1) continue;
+            var f = items[0];
             errors.Add(new("EquipmentSignalNameCollision",
-                $"{g.Count()} signals collide on OPC UA NodeId '{g.Key}' (equipment '{f.Eq}', name '{f.Name}'); " +
+                $"{items.Count} signals collide on OPC UA NodeId '{g.Key}' (equipment '{f.Eq}', name '{f.Name}'); " +
                 "a Name must be unique across Tag and VirtualTag within an equipment+folder",
                 f.Eq));
         }