fix(core): keep cut-offs in their cut sequence through regenerate and save

A cut-off's place in Plate.Parts is its cut sequence number, but
RegenerateCutOffs removed every cut-off part and appended it again, so
any part drag, fill or cut-off move sent the cut-offs to the end. The
nest file didn't store the position either, so reopening did the same.

RegenerateCutOffs now puts each cut-off back at its previous index (new
cut-offs go at the end), and CutOffDto.Sequence saves the index. Older
files without it load the cut-offs at the end, as before.

OpenNest.IO/NestFormat.cs

@@ -87,6 +87,12 @@ namespace OpenNest.IO
             public string Axis { get; init; } = "vertical";
             public double? StartLimit { get; init; }
             public double? EndLimit { get; init; }
+
+            /// <summary>
+            /// Zero-based place in the plate's cut sequence (<c>Plate.Parts</c> order,
+            /// cut-offs included). Null in older files, which load it at the end.
+            /// </summary>
+            public int? Sequence { get; init; }
         }
 
         public record SizeDto