fix: preserve leading rapid in programs to prevent missing contour segment

The CAD converter and BOM import were stripping the leading RapidMove
after normalizing program coordinates to origin. This left programs
starting with a LinearMove, causing the post-processor to use that
endpoint as the pierce point — making the first contour edge zero-length
and losing the closing segment (e.g. the bottom line on curved parts).

Root cause: CadConverterForm.GetDrawings(), OnSplitClicked(), and
BomImportForm all called pgm.Codes.RemoveAt(0) after offsetting the
rapid to origin. The rapid at (0,0) is a harmless no-op that marks the
contour start point for downstream processing.

Also adds EnsureLeadingRapid() safety net in the Cincinnati post for
existing nest files that already have the rapid stripped.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

OpenNest/Forms/BomImportForm.cs

@@ -432,7 +432,6 @@ namespace OpenNest.Forms
                             var rapid = (RapidMove)pgm[0];
                             drawing.Source.Offset = rapid.EndPoint;
                             pgm.Offset(-rapid.EndPoint);
-                            pgm.Codes.RemoveAt(0);
                         }
 
                         drawing.Program = pgm;

OpenNest/Forms/CadConverterForm.cs

@@ -362,7 +362,6 @@ namespace OpenNest.Forms
                 var rapid = (RapidMove)pgm[0];
                 originOffset = rapid.EndPoint;
                 pgm.Offset(-originOffset);
-                pgm.Codes.RemoveAt(0);
             }
 
             var drawing = new Drawing(item.Name, pgm);
@@ -660,7 +659,8 @@ namespace OpenNest.Forms
                     var rapid = (RapidMove)firstCode;
                     drawing.Source.Offset = rapid.EndPoint;
                     pgm.Offset(-rapid.EndPoint);
-                    pgm.Codes.RemoveAt(0);
+                    // Keep the rapid (now at origin) — it marks the contour
+                    // start and is needed by the post for correct pierce placement.
                 }
 
                 if (item == CurrentItem && programEditor.IsDirty && programEditor.Program != null)