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/DefaultNestEngine.cs

@@ -64,7 +64,7 @@ namespace OpenNest
             var best = context.CurrentBest ?? new List<Part>();
 
             if (item.Quantity > 0 && best.Count > item.Quantity)
-                best = ShrinkFiller.TrimToCount(best, item.Quantity, ShrinkAxis.Width);
+                best = ShrinkFiller.TrimToCount(best, item.Quantity, TrimAxis);
 
             ReportProgress(progress, new ProgressReport
             {