fix: add overlap safety check and diagnostics to FillGrid Step 2

FillGrid had no overlap check after perpendicular tiling of the row
pattern (Step 2), unlike Step 1 which had one. When geometry-aware
FindPatternCopyDistance underestimated row spacing, overlapping parts
were returned unchecked.

Changes:
- Make FillLinear.HasOverlappingParts shape-aware (bbox pre-filter +
  Part.Intersects) instead of bbox-only, preventing false positives on
  interlocking pairs while catching real overlaps
- Add missing overlap safety check after Step 2 perpendicular tiling
  with bbox fallback
- Add diagnostic Debug.WriteLine logging when overlap fallback triggers,
  including engine label, step, direction, work area, spacing, pattern
  details, and overlapping part locations/rotations for reproduction
- Add FillLinear.Label property set at all callsites for log traceability
- Refactor LinearFillStrategy and ExtentsFillStrategy to use shared
  FillHelpers.BestOverAngles helper for angle-sweep logic

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

OpenNest.Engine/Strategies/ExtentsFillStrategy.cs

@@ -20,22 +20,10 @@ namespace OpenNest.Engine.Strategies
 
             var angles = new[] { bestRotation, bestRotation + Angle.HalfPI };
 
-            List<Part> best = null;
-            var comparer = context.Policy?.Comparer ?? new DefaultFillComparer();
-
-            foreach (var angle in angles)
-            {
-                context.Token.ThrowIfCancellationRequested();
-                var result = filler.Fill(context.Item.Drawing, angle,
-                    context.PlateNumber, context.Token, context.Progress);
-                if (result != null && result.Count > 0)
-                {
-                    if (best == null || comparer.IsBetter(result, best, context.WorkArea))
-                        best = result;
-                }
-            }
-
-            return best ?? new List<Part>();
+            return FillHelpers.BestOverAngles(context, angles,
+                angle => filler.Fill(context.Item.Drawing, angle,
+                    context.PlateNumber, context.Token, context.Progress),
+                NestPhase.Extents, "Extents");
         }
     }
 }