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/Circle.cs

@@ -263,7 +263,7 @@ namespace OpenNest.Geometry
             boundingBox.X = Center.X - Radius;
             boundingBox.Y = Center.Y - Radius;
             boundingBox.Width = Diameter;
-            boundingBox.Height = Diameter;
+            boundingBox.Length = Diameter;
         }
 
         public override Entity OffsetEntity(double distance, OffsetSide side)