feat(uns): area + line CRUD with #122 reassignment guard

src/Server/ZB.MOM.WW.OtOpcUa.AdminUI/Uns/UnsMutationResult.cs

@@ -0,0 +1,11 @@
+namespace ZB.MOM.WW.OtOpcUa.AdminUI.Uns;
+
+/// <summary>
+/// Outcome of a UNS structural mutation (create/update/delete of an area or line).
+/// On success <see cref="Ok"/> is <c>true</c> and <see cref="Error"/> is <c>null</c>;
+/// on a guarded or concurrency failure <see cref="Ok"/> is <c>false</c> and
+/// <see cref="Error"/> carries the operator-facing message the caller should surface.
+/// </summary>
+/// <param name="Ok">Whether the mutation was applied.</param>
+/// <param name="Error">The operator-facing failure message, or <c>null</c> on success.</param>
+public readonly record struct UnsMutationResult(bool Ok, string? Error);