perf: optimize fill hot path — bbox pre-check and geometry inner loop

- Add bounding box rejection in HasOverlaps to skip expensive
  Part.Intersects (CNC→geometry conversion) for non-adjacent parts.
  Eliminates ~35% CPU in IsBetterValidFill for grid layouts.
- Optimize RayEdgeDistance: access Line fields directly instead of
  property getters (avoids Vector struct copies), inline IsEqualTo
  with direct range comparison (avoids Math.Abs), and precompute
  deltas for reuse in interpolation.
- Cache line endpoints in DirectionalDistance outer loop to avoid
  repeated struct copies in the inner loop.
- Add fill timer to ActionClone.Fill, displayed in PlateView status
  bar as "Fill: N parts in M ms".

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

OpenNest.Engine/NestEngine.cs

@@ -273,8 +273,19 @@ namespace OpenNest
 
             for (var i = 0; i < parts.Count; i++)
             {
+                var box1 = parts[i].BoundingBox;
+
                 for (var j = i + 1; j < parts.Count; j++)
                 {
+                    var box2 = parts[j].BoundingBox;
+
+                    // Fast bounding box rejection — if boxes don't overlap,
+                    // the parts can't intersect. Eliminates nearly all pairs
+                    // in grid layouts.
+                    if (box1.Right < box2.Left || box2.Right < box1.Left ||
+                        box1.Top < box2.Bottom || box2.Top < box1.Bottom)
+                        continue;
+
                     List<Vector> pts;
 
                     if (parts[i].Intersects(parts[j], out pts))