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.Core/Geometry/Line.cs

@@ -6,8 +6,8 @@ namespace OpenNest.Geometry
 {
     public class Line : Entity
     {
-        private Vector pt1;
-        private Vector pt2;
+        internal Vector pt1;
+        internal Vector pt2;
 
         public Line()
         {