fix: allow line-on-line contact and remove extra spacing gap

- Part.Intersects: filter intersection points at a vertex of either
  shape (was both), so edge-touching parts are not flagged as overlapping
- NestEngineBase.HasOverlaps: use epsilon-based bounding box pre-filter
  consistent with FillExtents and Plate.HasOverlappingParts
- PartGeometry.GetOffsetPartLines: remove extra chordTolerance added to
  spacing offset — was causing 0.002" gap beyond the intended part spacing

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

OpenNest.Core/PartGeometry.cs

@@ -45,7 +45,7 @@ namespace OpenNest
             var profile = new ShapeProfile(
                 entities.Where(e => e.Layer != SpecialLayers.Rapid).ToList());
             var lines = new List<Line>();
-            var totalSpacing = spacing + chordTolerance;
+            var totalSpacing = spacing;
 
             AddOffsetLines(lines, profile.Perimeter.OffsetOutward(totalSpacing),
                 chordTolerance, part.Location);
@@ -63,7 +63,7 @@ namespace OpenNest
             var profile = new ShapeProfile(
                 entities.Where(e => e.Layer != SpecialLayers.Rapid).ToList());
             var lines = new List<Line>();
-            var totalSpacing = spacing + chordTolerance;
+            var totalSpacing = spacing;
 
             AddOffsetDirectionalLines(lines, profile.Perimeter.OffsetOutward(totalSpacing),
                 chordTolerance, part.Location, facingDirection);
@@ -97,7 +97,7 @@ namespace OpenNest
             var profile = new ShapeProfile(
                 entities.Where(e => e.Layer != SpecialLayers.Rapid).ToList());
             var lines = new List<Line>();
-            var totalSpacing = spacing + chordTolerance;
+            var totalSpacing = spacing;
 
             AddOffsetDirectionalLines(lines, profile.Perimeter.OffsetOutward(totalSpacing),
                 chordTolerance, part.Location, facingDirection);