fix: correct Size width/length axis mapping throughout codebase

The Size fix (d4222db) changed Size.Width to Y axis and Size.Length to
X axis but only updated DrawPlate/LayoutViewGL. BoundingBox, WorkArea,
rotations, DXF export, and engine code still used the old Width=X
convention, causing the fill engine to get a swapped work area (60x120
instead of 120x60) and parts to fill in the wrong direction.

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

OpenNest.Engine/DefaultNestEngine.cs

@@ -71,7 +71,7 @@ namespace OpenNest
 
             // Top pair candidates — check if pairs tile better in this box.
             var bestFits = BestFitCache.GetOrCompute(
-                drawing, Plate.Size.Width, Plate.Size.Length, Plate.PartSpacing);
+                drawing, Plate.Size.Length, Plate.Size.Width, Plate.PartSpacing);
             var topPairs = bestFits.Where(r => r.Keep).Take(3);
 
             foreach (var pair in topPairs)