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.Engine/FillScore.cs

@@ -80,7 +80,7 @@ namespace OpenNest
             if (maxRight < workArea.Right)
             {
                 var width = workArea.Right - maxRight;
-                var height = workArea.Height;
+                var height = workArea.Length;
 
                 if (System.Math.Min(width, height) >= MinRemnantDimension)
                     largest = System.Math.Max(largest, width * height);