feat(io): save and reload display perimeters

A display perimeter written without its own tag reloaded as an ordinary
cut, so a saved blank would come back with its outline to be cut. Programs
now carry it as :DISPLAYPERIMETER and the reader maps it back, and stored
source entities keep the DISPLAY PERIMETER layer so editing the drawing
rebuilds the same never-cut outline.

Builds older than this read the unknown tag as a cut. Open nests with
display perimeters only in builds that include this change.

OpenNest.IO/ProgramReader.cs

@@ -288,6 +288,10 @@ namespace OpenNest.IO
                                         layer = LayerType.Tab;
                                         break;
 
+                                    case "DISPLAYPERIMETER":
+                                        layer = LayerType.DisplayPerimeter;
+                                        break;
+
                                     case "SUPPRESSED":
                                         suppressed = true;
                                         break;
@@ -394,6 +398,10 @@ namespace OpenNest.IO
                                         layer = LayerType.Tab;
                                         break;
 
+                                    case "DISPLAYPERIMETER":
+                                        layer = LayerType.DisplayPerimeter;
+                                        break;
+
                                     case "SUPPRESSED":
                                         suppressed = true;
                                         break;