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/Line.cs

@@ -381,12 +381,12 @@ namespace OpenNest.Geometry
             if (StartPoint.Y < EndPoint.Y)
             {
                 boundingBox.Y = StartPoint.Y;
-                boundingBox.Height = EndPoint.Y - StartPoint.Y;
+                boundingBox.Length = EndPoint.Y - StartPoint.Y;
             }
             else
             {
                 boundingBox.Y = EndPoint.Y;
-                boundingBox.Height = StartPoint.Y - EndPoint.Y;
+                boundingBox.Length = StartPoint.Y - EndPoint.Y;
             }
         }