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.Engine/Fill/PartBoundary.cs

@@ -34,19 +34,16 @@ namespace OpenNest.Engine.Fill
 
             if (perimeter != null)
             {
-                var offsetEntity = perimeter.OffsetOutward(spacing);
-
-                if (offsetEntity != null)
-                {
-                    // Circumscribe arcs so polygon vertices are always outside
-                    // the true arc — guarantees the boundary never under-estimates.
-                    var polygon = offsetEntity.ToPolygonWithTolerance(
-                        PolygonTolerance,
-                        circumscribe: true
-                    );
-                    polygon.RemoveSelfIntersections();
-                    _polygons.Add(polygon);
-                }
+                // Conservative offset: the boundary never under-estimates the spacing.
+                // Holes appear only where the perimeter curls back on itself.
+                var offset = ClipperBridge.OffsetPerimeter(
+                    perimeter,
+                    spacing,
+                    PolygonTolerance,
+                    circumscribe: true
+                );
+                _polygons.AddRange(offset.Outers);
+                _polygons.AddRange(offset.Holes);
             }
 
             PrecomputeDirectionalEdges(