fix: resolve grid overlap bug and parallelize fill loops

The push algorithm's copy distance formula (bboxDim - slideDistance)
produced distances smaller than the part width when inflated boundary
arc vertices interacted spuriously, causing ~0.05 unit overlaps between
all adjacent grid parts.

Two fixes applied:
- Clamp ComputeCopyDistance to bboxDim + PartSpacing minimum
- Use circumscribed polygons (R/cos(halfStep)) for PartBoundary arc
  discretization so chord segments never cut inside the true arc,
  eliminating the ChordTolerance offset workaround

Also parallelized three sequential fill loops using Parallel.ForEach:
- FindBestFill angle sweep (up to 38 angles x 2 directions)
- FillPattern angle sweep for group/pair fills
- FillRemainingStrip rotation loop

Added diagnostic logging to HasOverlaps, FindCopyDistance, and
FillRecursive for debugging fill issues.

Test result: 45 parts @ 79.6% -> 47 parts @ 83.1%, zero overlaps.

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

OpenNest.Core/Geometry/Arc.cs

@@ -203,20 +203,24 @@ namespace OpenNest.Geometry
         /// </summary>
         /// <param name="segments">Number of parts to divide the arc into.</param>
         /// <returns></returns>
-        public List<Vector> ToPoints(int segments = 1000)
+        public List<Vector> ToPoints(int segments = 1000, bool circumscribe = false)
         {
             var points = new List<Vector>();
             var stepAngle = reversed
                 ? -SweepAngle() / segments
                 : SweepAngle() / segments;
 
+            var r = circumscribe && segments > 0
+                ? Radius / System.Math.Cos(System.Math.Abs(stepAngle) / 2.0)
+                : Radius;
+
             for (int i = 0; i <= segments; ++i)
             {
                 var angle = stepAngle * i + StartAngle;
 
                 points.Add(new Vector(
-                    System.Math.Cos(angle) * Radius + Center.X,
-                    System.Math.Sin(angle) * Radius + Center.Y));
+                    System.Math.Cos(angle) * r + Center.X,
+                    System.Math.Sin(angle) * r + Center.Y));
             }
 
             return points;