fix: leave etch/scribe marks out of nesting geometry

Every nesting-geometry consumer filtered only rapids, so scribe/etch
moves counted as part material. An etch tick that ends a hair outside
the outline (PEP bend ticks start on the notch edge) made the part
"open geometry leaving the material region": the job validator threw
and every built-in engine plus Gpt6Astra crashed on real PEP jobs
(PT75, drawing 4980 A01 PT77). Marks are only on the surface, so they
should never affect placement, collision, area, or validation.

- SpecialLayers.IsMaterial excludes Rapid and Scribe; used by drawing
  area, canonical angle, part collision, PartGeometry, plate perimeter,
  best-fit/pair evaluation, rotation analysis, GPU evaluators, and both
  validators. Timing, display, splitting and posts still see marks.
- ConvertGeometry also maps the saved SCRIBE layer name to Scribe, so
  programs rebuilt from stored entities keep their marks.
- NestReader repairs older files (e.g. PepNestExport output) whose
  programs saved etch as cut moves while source entities kept SCRIBE.

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

OpenNest.Core/SpecialLayers.cs

@@ -18,5 +18,12 @@ namespace OpenNest
         public static readonly Layer Leadout = new Layer("LEADOUT") { Color = Color.Brown };
 
         public static readonly Layer Scribe = new Layer("SCRIBE") { Color = Color.Magenta };
+
+        /// <summary>
+        /// True when an entity converted from a part program describes part material. Rapids
+        /// and scribe/etch marks are excluded: marks are only on the surface, so they never
+        /// bound material and must not affect nesting, collision, area, or validation.
+        /// </summary>
+        public static bool IsMaterial(Layer layer) => layer != Rapid && layer != Scribe;
     }
 }