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/BestFit/PairEvaluator.cs

@@ -57,7 +57,7 @@ namespace OpenNest.Engine.BestFit
                 var combinedBox = ((IEnumerable<IBoundable>)new IBoundable[] { part1, part2 }).GetBoundingBox();
                 bestArea = combinedBox.Area();
                 bestWidth = combinedBox.Width;
-                bestHeight = combinedBox.Height;
+                bestHeight = combinedBox.Length;
                 bestRotation = 0;
             }