refactor: rename Size.Height to Size.Length across codebase

"Length" is more natural than "height" for flat plate materials.
Renames the field on OpenNest.Geometry.Size, Box.Height property,
and all references across 38 files.

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

OpenNest.Core/Geometry/BoundingBox.cs

@@ -13,7 +13,7 @@ namespace OpenNest.Geometry
             double minX = boxes[0].X;
             double minY = boxes[0].Y;
             double maxX = boxes[0].X + boxes[0].Width;
-            double maxY = boxes[0].Y + boxes[0].Height;
+            double maxY = boxes[0].Y + boxes[0].Length;
 
             foreach (var box in boxes)
             {