fix: improve circle nesting with curve-to-curve distance and min copy spacing

Add Phase 3 curve-to-curve direct distance in CpuDistanceComputer to
catch contacts that vertex sampling misses between curved entities.
Enforce minimum copy distance in FillLinear to prevent bounding box
overlap when circumscribed polygon boundaries overshoot true arcs.

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

OpenNest.Engine/Fill/FillLinear.cs

@@ -119,10 +119,11 @@ namespace OpenNest.Engine.Fill
             var maxCopyDistance = FindMaxPairDistance(
                 patternA.Parts, boundaries, offset, pushDir, opposite, startOffset);
 
-            if (maxCopyDistance < Tolerance.Epsilon)
-                return bboxDim + PartSpacing;
-
-            return maxCopyDistance;
+            // The copy distance must be at least bboxDim + PartSpacing to prevent
+            // bounding box overlap. Cross-pair slides can underestimate when the
+            // circumscribed polygon boundary overshoots the true arc, creating
+            // spurious contacts between diagonal parts in adjacent copies.
+            return System.Math.Max(maxCopyDistance, bboxDim + PartSpacing);
         }
 
         /// <summary>