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/Controls/PlateView.cs

@@ -139,7 +139,7 @@ namespace OpenNest.Controls
         public string Status
         {
             get { return status; }
-            protected set
+            set
             {
                 status = value;