fix: prevent PushBoundingBox from making obstacles invisible to geometry push

PushBoundingBox left exactly partSpacing between bounding boxes, but the
geometry push inflates offset lines by halfSpacing + chordTolerance per
side (totaling partSpacing + 2*chordTolerance). The 0.002 overlap caused
RayEdgeDistance to return MaxValue for negative t, making the nearest
obstacle invisible and allowing the part to push through it.

Subtract 2*ChordTolerance from the BB push distance so the gap is wide
enough for the offset geometry lines to remain non-overlapping.

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

OpenNest.Engine/Fill/Compactor.cs

@@ -157,7 +157,7 @@ namespace OpenNest.Engine.Fill
                         continue;
 
                     var gap = SpatialQuery.DirectionalGap(movingBox, obstacleBoxes[i], direction);
-                    var d = gap - partSpacing;
+                    var d = gap - partSpacing - 2 * ChordTolerance;
                     if (d < 0) d = 0;
                     if (d < distance)
                         distance = d;