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/BestFit/PolygonHelper.cs

@@ -26,16 +26,15 @@ namespace OpenNest.Engine.BestFit
             if (perimeter == null)
                 return new PolygonExtractionResult(null, Vector.Zero);
 
-            // Ensure CW winding for correct outward offset direction.
-            definedShape.NormalizeWinding();
+            // Circumscribe so the polygon never under-estimates the part (or its offset).
+            var polygon =
+                halfSpacing > 0
+                    ? ClipperBridge
+                        .OffsetPerimeter(perimeter, halfSpacing, 0.01, circumscribe: true)
+                        .LargestOuter()
+                    : perimeter.ToPolygonWithTolerance(0.01, circumscribe: true);
 
-            var inflated =
-                halfSpacing > 0 ? (perimeter.OffsetOutward(halfSpacing) ?? perimeter) : perimeter;
-
-            // Convert to polygon with circumscribed arcs for tight nesting.
-            var polygon = inflated.ToPolygonWithTolerance(0.01, circumscribe: true);
-
-            if (polygon.Vertices.Count < 3)
+            if (polygon == null || polygon.Vertices.Count < 3)
                 return new PolygonExtractionResult(null, Vector.Zero);
 
             // Normalize: move polygon to origin.