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/Sequencing/AdvancedSequencer.cs

@@ -28,7 +28,7 @@ namespace OpenNest.Engine.Sequencing
             rows.Sort((a, b) => a.RowY.CompareTo(b.RowY));
 
             // Determine initial direction based on exit point
-            var leftToRight = exit.X > plate.Size.Width * 0.5;
+            var leftToRight = exit.X > plate.Size.Length * 0.5;
 
             var result = new List<SequencedPart>(parts.Count);
             foreach (var row in rows)