refactor(geometry): move polygon offset callers onto ClipperBridge

Per-entity offsetting left spikes and inverted loops wherever a feature is
narrower than the spacing (1.nest), and RemoveSelfIntersections only
caught proper crossings. The callers that already flatten to polygons now
take a single Clipper region offset instead:

- PolygonHelper (BestFit) and PartBoundary use the conservative mode, which
  keeps their never-under-estimate guarantee. PartBoundary also keeps holes
  that appear when a perimeter curls back on itself.
- NestValidator offsets perimeter and cutouts in one region; Clipper drops
  collapsed cutouts, so the collapsed-or-flipped heuristic goes away.
- CutOff.IntersectPerimeter offsets through the bridge. The old
  OffsetEntity(Left) grew CW perimeters but shrank CCW ones, so with the
  plate's perimeter cache a cut-off ran through the part; slots narrower
  than twice the clearance now close up instead of leaving a gap.
- GetOffsetPartLines (3 overloads) and the AddOffset* helpers had no
  callers and are removed, as is EntityView's never-defined DRAW_OFFSET
  block.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

OpenNest.Core/CutOff.cs

@@ -13,6 +13,8 @@ namespace OpenNest
 
     public class CutOff
     {
+        private const double OffsetTolerance = 0.001;
+
         public Vector Position { get; set; }
         public CutOffAxis Axis { get; set; }
         public double? StartLimit { get; set; }
@@ -163,14 +165,23 @@ namespace OpenNest
             double clearance
         )
         {
-            var target = OffsetOutward(perimeter, clearance) ?? perimeter;
-            var usedOffset = target != perimeter;
+            var offset = OffsetOutward(perimeter, clearance);
+            var usedOffset = offset != null;
+            var targets = offset ?? new List<Entity> { perimeter };
             var cutLine = new Line(
                 MakePoint(cutPosition, lineStart),
                 MakePoint(cutPosition, lineEnd)
             );
 
-            if (!target.Intersects(cutLine, out var pts) || pts.Count < 2)
+            var pts = new List<Vector>();
+
+            foreach (var target in targets)
+            {
+                if (target.Intersects(cutLine, out var targetPts))
+                    pts.AddRange(targetPts);
+            }
+
+            if (pts.Count < 2)
                 return null;
 
             var coords = pts.Select(pt => Axis == CutOffAxis.Vertical ? pt.Y : pt.X)
@@ -188,21 +199,37 @@ namespace OpenNest
             return result;
         }
 
-        private static Entity OffsetOutward(Entity perimeter, double clearance)
+        /// <summary>
+        /// Grows the perimeter by the clearance as one Clipper region offset, so slots
+        /// narrower than twice the clearance close up instead of leaving a gap the cut
+        /// could run into. Holes appear only where the perimeter curls back on itself.
+        /// </summary>
+        private static List<Entity> OffsetOutward(Entity perimeter, double clearance)
         {
             if (clearance <= 0)
                 return null;
 
-            try
-            {
-                var offset = perimeter.OffsetEntity(clearance, OffsetSide.Left);
-                offset?.UpdateBounds();
-                return offset;
-            }
-            catch
+            var offset = perimeter switch
             {
+                Shape shape => ClipperBridge.OffsetPerimeter(
+                    shape,
+                    clearance,
+                    OffsetTolerance,
+                    circumscribe: true
+                ),
+                Polygon polygon => ClipperBridge.OffsetPerimeter(
+                    polygon,
+                    clearance,
+                    OffsetTolerance,
+                    circumscribe: true
+                ),
+                _ => null,
+            };
+
+            if (offset == null || offset.Outers.Count == 0)
                 return null;
-            }
+
+            return offset.Outers.Concat(offset.Holes).Cast<Entity>().ToList();
         }
 
         private Vector MakePoint(double cutCoord, double lineCoord) =>