feat(cad): cut breakoff joining lines as tabbed open break cuts

Breakoffs are now picked by their joining line, default to the smaller side as scrap
(flippable), and cut that line between tabs on a new Break layer instead of slots.
Plan Cutting and Assign Lead-ins enter each cut from the scrap with the nest's internal
lead-in, or pierce on the line without one; checks treat break cuts as intentional open
cuts that are not material.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

OpenNest.IO/ProgramReader.cs

@@ -280,6 +280,10 @@ namespace OpenNest.IO
                                         layer = LayerType.Scribe;
                                         break;
 
+                                    case "BREAK":
+                                        layer = LayerType.Break;
+                                        break;
+
                                     case "SUPPRESSED":
                                         suppressed = true;
                                         break;
@@ -378,6 +382,10 @@ namespace OpenNest.IO
                                         layer = LayerType.Scribe;
                                         break;
 
+                                    case "BREAK":
+                                        layer = LayerType.Break;
+                                        break;
+
                                     case "SUPPRESSED":
                                         suppressed = true;
                                         break;