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/HorizontalRemnantEngine.cs

@@ -24,6 +24,8 @@ namespace OpenNest
 
         public override NestDirection? PreferredDirection => NestDirection.Vertical;
 
+        public override ShrinkAxis TrimAxis => ShrinkAxis.Length;
+
         public override List<double> BuildAngles(NestItem item, double bestRotation, Box workArea)
         {
             var baseAngles = new List<double> { bestRotation, bestRotation + Angle.HalfPI };