feat: engine-specific TrimAxis and rename ShrinkAxis.Height to Length

Make quantity trimming direction-aware: DefaultNestEngine uses TrimAxis
(virtual property on NestEngineBase) so HorizontalRemnantEngine removes
topmost parts instead of rightmost. Rename ShrinkAxis.Height → Length
for consistency with Width/Length naming used throughout the codebase.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

OpenNest.Engine/NestEngineBase.cs

@@ -43,6 +43,8 @@ namespace OpenNest
 
         public virtual NestDirection? PreferredDirection => null;
 
+        public virtual ShrinkAxis TrimAxis => ShrinkAxis.Width;
+
         public virtual List<double> BuildAngles(NestItem item, double bestRotation, Box workArea)
         {
             return new List<double> { bestRotation, bestRotation + OpenNest.Math.Angle.HalfPI };