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/Polygon.cs

@@ -312,7 +312,7 @@ namespace OpenNest.Geometry
             boundingBox.X = minX;
             boundingBox.Y = minY;
             boundingBox.Width = maxX - minX;
-            boundingBox.Height = maxY - minY;
+            boundingBox.Length = maxY - minY;
         }
 
         public override Entity OffsetEntity(double distance, OffsetSide side)