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/Gpt6AstraNestingEngine.cs

@@ -22,7 +22,7 @@ public sealed class Gpt6AstraNestingEngine : INestingEngine
         var best = initial;
         Plan? complete = IsComplete(initial) ? initial : null;
         var trials = new Dictionary<string, SheetTrial>(StringComparer.Ordinal);
-        var priorities = job.Parts.Select(p => p.Priority).Distinct().OrderDescending().ToArray();
+        var priorities = job.Parts.Select(p => p.Priority).Distinct().Order().ToArray();
         var evaluated = 0;
         var unitCosts = Enumerable.Repeat(double.PositiveInfinity, parts.Length).ToArray();
         while (frontier.Count > 0)