fix(gpt6astra): rank lower Priority first and drop etch marks from geometry

Priority was sorted descending, the reverse of the host (StockLadder and
NestJobCandidateComparer treat a lower number as more important), so a
priority-9 part beat a priority-0 part for scarce stock. The existing test
encoded the inverted rule and now asserts the host's direction.

Part geometry filtered only rapids, so scribe/etch moves counted as
material - the bug OpenNest fixed in 1b5e1b1. Use SpecialLayers.IsMaterial.

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

OpenNest.Engine.Gpt6Astra/PreparedGeometry.cs

@@ -78,7 +78,7 @@ internal static class GeometryPreparation
         {
             token.ThrowIfCancellationRequested();
             var entities = ConvertProgram.ToGeometry(DrawingJobMapper.ToProgram(part.Geometry))
-                .Where(e => !ReferenceEquals(e.Layer, SpecialLayers.Rapid)).ToList();
+                .Where(e => SpecialLayers.IsMaterial(e.Layer)).ToList();
             // Input validation has established that open marks lie inside material. They
             // must not be interpreted as holes by ShapeProfile.
             var closed = ShapeBuilder.GetShapes(entities).Where(s => s.IsClosed())