fix: use chain tolerance for shape building to handle DXF endpoint gaps

DXF files can have endpoint gaps at entity junctions that fall right at
the floating-point boundary of Tolerance.Epsilon (0.00001). This caused
shapes to not close, resulting in 0 area and 0% utilization in Best-Fit.

Added ChainTolerance (0.0001) for endpoint chaining in GetConnected and
Shape.IsClosed, keeping the tighter Epsilon for geometric precision.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

OpenNest.Core/Math/Tolerance.cs

@@ -6,6 +6,12 @@ namespace OpenNest.Math
     {
         public const double Epsilon = 0.00001;
 
+        /// <summary>
+        /// Tolerance for chaining entity endpoints when building shapes from DXF imports.
+        /// Larger than Epsilon to handle floating-point gaps at entity junctions.
+        /// </summary>
+        public const double ChainTolerance = 0.0001;
+
         public static bool IsEqualTo(this double a, double b, double tolerance = Epsilon)
         {
             return System.Math.Abs(b - a) <= tolerance;