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/FillExtents.cs

@@ -47,13 +47,21 @@ namespace OpenNest.Engine.Fill
 
             var adjusted = AdjustColumn(pair.Value, column, token);
 
+            // The iterative pair adjustment can shift parts enough to cause
+            // genuine overlap. Fall back to the unadjusted column when this happens.
+            if (HasOverlappingParts(adjusted))
+            {
+                Debug.WriteLine("[FillExtents] Adjusted column has overlaps, using unadjusted");
+                adjusted = column;
+            }
+
             NestEngineBase.ReportProgress(progress, new ProgressReport
             {
                 Phase = NestPhase.Extents,
                 PlateNumber = plateNumber,
                 Parts = adjusted,
                 WorkArea = workArea,
-                Description = $"Extents: adjusted column {adjusted.Count} parts",
+                Description = $"Extents: column {adjusted.Count} parts",
             });
 
             var result = RepeatColumns(adjusted, token);
@@ -386,5 +394,31 @@ namespace OpenNest.Engine.Fill
                    part.BoundingBox.Left >= workArea.Left - Tolerance.Epsilon &&
                    part.BoundingBox.Bottom >= workArea.Bottom - Tolerance.Epsilon;
         }
+
+        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;
+        }
     }
 }