feat: dual-tangent arc fitting and DXF version export

Add ArcFit.FitWithDualTangent to constrain replacement arcs to match
tangent directions at both endpoints, preventing kinks without
introducing gaps. Add DXF year selection to CAD converter export.

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

OpenNest.Tests/GeometrySimplifierTests.cs

@@ -1,4 +1,7 @@
 using OpenNest.Geometry;
+using OpenNest.IO;
+using System.IO;
+using System.Linq;
 using Xunit;
 
 namespace OpenNest.Tests;
@@ -127,4 +130,52 @@ public class GeometrySimplifierTests
         // Index 6 is the fitted arc replacing the second run
         Assert.IsType<Arc>(result.Entities[6]);
     }
+
+    [Fact]
+    public void Apply_DynaPanDxf_NoGapsAfterSimplification()
+    {
+        var path = @"C:\Users\aisaacs\Desktop\Sullys Q29 DXFs\SULLYS-031 Dyna Pan.dxf";
+        if (!File.Exists(path))
+            return; // skip if file not available
+
+        var importer = new DxfImporter();
+        var result = importer.Import(path);
+        var shapes = ShapeBuilder.GetShapes(result.Entities);
+
+        var simplifier = new GeometrySimplifier { Tolerance = 0.004 };
+
+        foreach (var shape in shapes)
+        {
+            var candidates = simplifier.Analyze(shape);
+            if (candidates.Count == 0) continue;
+
+            var simplified = simplifier.Apply(shape, candidates);
+
+            // Check for gaps between consecutive entities
+            for (var i = 0; i < simplified.Entities.Count - 1; i++)
+            {
+                var current = simplified.Entities[i];
+                var next = simplified.Entities[i + 1];
+
+                var currentEnd = current switch
+                {
+                    Line l => l.EndPoint,
+                    Arc a => a.EndPoint(),
+                    _ => Vector.Invalid
+                };
+                var nextStart = next switch
+                {
+                    Line l => l.StartPoint,
+                    Arc a => a.StartPoint(),
+                    _ => Vector.Invalid
+                };
+
+                if (!currentEnd.IsValid() || !nextStart.IsValid()) continue;
+
+                var gap = currentEnd.DistanceTo(nextStart);
+                Assert.True(gap < 0.005,
+                    $"Gap of {gap:F4} between entities {i} ({current.GetType().Name}) and {i + 1} ({next.GetType().Name})");
+            }
+        }
+    }
 }