fix(opus55): cut clearance margin to 0.003 and drop etch marks from geometry

The 0.022 margin assumed validators flatten arcs at 0.01; NestValidator
uses 0.001, so a pair can read at most 0.002 closer than true. 0.003
covers that plus the 1e-4 Clipper grid, returning ~0.019 per gap. Arc,
disc, obround and zero-spacing tests still pass.

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.Opus55/PartCatalog.cs

@@ -111,7 +111,7 @@ internal static class PartCatalog
     {
         var entities = ConvertProgram
             .ToGeometry(DrawingJobMapper.ToProgram(geometry))
-            .Where(e => !ReferenceEquals(e.Layer, SpecialLayers.Rapid))
+            .Where(e => SpecialLayers.IsMaterial(e.Layer))
             .ToList();
         if (entities.Count == 0)
             return null;