feat: separate DXF entities onto per-type layers, recognize SCRIBE marker

DrawLoop previously drew every entity onto the single loop layer, so
scribe/display marks were indistinguishable from cuts in the exported
DXF. Non-cut entity types now get their own layer, and ProgramReader
now parses the SCRIBE marker into EntityType.Scribe so scribed moves
route to their own layer instead of falling through as untyped cuts.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

PepLib.Core/IO/ProgramReader.cs

@@ -189,6 +189,10 @@ namespace PepLib.IO
                                 case "DISPLAY":
                                     type = EntityType.Display;
                                     break;
+
+                                case "SCRIBE":
+                                    type = EntityType.Scribe;
+                                    break;
                             }
                             break;
                         }
@@ -266,6 +270,10 @@ namespace PepLib.IO
                                 case "DISPLAY":
                                     type = EntityType.Display;
                                     break;
+
+                                case "SCRIBE":
+                                    type = EntityType.Scribe;
+                                    break;
                             }
                             break;
                         }