refactor(engine): delete obsolete StripNestResult and StripDirection

Both types were only used internally by the old StripNestEngine.Nest
strip-orientation logic, which has been replaced by IterativeShrinkFiller.
No references remain outside of these files.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

OpenNest.Engine/StripDirection.cs

@@ -1,8 +0,0 @@
-namespace OpenNest
-{
-    public enum StripDirection
-    {
-        Bottom,
-        Left
-    }
-}

OpenNest.Engine/StripNestResult.cs

@@ -1,14 +0,0 @@
-using OpenNest.Engine.Fill;
-using OpenNest.Geometry;
-using System.Collections.Generic;
-
-namespace OpenNest
-{
-    internal class StripNestResult
-    {
-        public List<Part> Parts { get; set; } = new();
-        public Box StripBox { get; set; }
-        public FillScore Score { get; set; }
-        public StripDirection Direction { get; set; }
-    }
-}