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precision parameter is obsolete.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.Tests/Geometry/EllipseConverterTests.cs

@@ -228,7 +228,7 @@ public class EllipseConverterTests
             using (var writer = new ACadSharp.IO.DxfWriter(stream, doc, false))
                 writer.Write();
 
-            var importer = new OpenNest.IO.DxfImporter { SplinePrecision = 200 };
+            var importer = new OpenNest.IO.DxfImporter();
             var result = importer.Import(tempPath);
 
             var arcCount = result.Entities.Count(e => e is Arc);