fix: remove import spline precision setting entirely

Spline import now uses SplineConverter (arc-based) so the configurable
precision parameter is obsolete. Removed the setting from the options
dialog, DxfImporter property, Settings files, and all callsites.
Hardcoded 200 as the sampling density for the intermediate point
evaluation that feeds into SplineConverter.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

OpenNest.IO/Extensions.cs

@@ -57,7 +57,7 @@ namespace OpenNest.IO
             return result;
         }
 
-        public static List<Geometry.Entity> ToOpenNest(this Spline spline, int precision)
+        public static List<Geometry.Entity> ToOpenNest(this Spline spline)
         {
             var layer = spline.Layer.ToOpenNest();
             var color = spline.ResolveColor();
@@ -67,7 +67,7 @@ namespace OpenNest.IO
             List<XYZ> curvePoints;
             try
             {
-                curvePoints = spline.PolygonalVertexes(precision > 0 ? precision : 200);
+                curvePoints = spline.PolygonalVertexes(200);
             }
             catch (Exception ex)
             {