fix: add overlap validation guards to FillExtents and StripeFiller

FillExtents falls back to the unadjusted column when iterative pair
adjustment shifts parts enough to cause genuine overlap. StripeFiller
rejects grid results where bounding boxes overlap, which can occur when
angle convergence produces slightly off-axis rotations.

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

OpenNest.Engine/Fill/StripeFiller.cs

@@ -158,6 +158,15 @@ public class StripeFiller
         if (gridParts.Count == 0)
             return null;
 
+        // Reject results where bounding boxes overlap — the angle convergence
+        // can produce slightly off-axis rotations where FillLinear's copy
+        // distance calculation doesn't fully account for the rotated geometry.
+        if (HasOverlappingParts(gridParts))
+        {
+            Debug.WriteLine($"[StripeFiller] Rejected grid: overlapping bounding boxes detected");
+            return null;
+        }
+
         var allParts = new List<Part>(gridParts);
 
         var remnantParts = FillRemnant(gridParts, primaryAxis);
@@ -470,4 +479,34 @@ public class StripeFiller
     {
         return axis == NestDirection.Horizontal ? box.Width : box.Length;
     }
+
+    /// <summary>
+    /// Checks if any pair of parts geometrically overlap. Uses bounding box
+    /// pre-filtering for performance, then falls back to shape intersection.
+    /// </summary>
+    private static bool HasOverlappingParts(List<Part> parts)
+    {
+        for (var i = 0; i < parts.Count; i++)
+        {
+            var b1 = parts[i].BoundingBox;
+
+            for (var j = i + 1; j < parts.Count; j++)
+            {
+                var b2 = parts[j].BoundingBox;
+
+                var overlapX = System.Math.Min(b1.Right, b2.Right)
+                             - System.Math.Max(b1.Left, b2.Left);
+                var overlapY = System.Math.Min(b1.Top, b2.Top)
+                             - System.Math.Max(b1.Bottom, b2.Bottom);
+
+                if (overlapX <= Tolerance.Epsilon || overlapY <= Tolerance.Epsilon)
+                    continue;
+
+                if (parts[i].Intersects(parts[j], out _))
+                    return true;
+            }
+        }
+
+        return false;
+    }
 }