feat: move PushDirection enum to OpenNest.Core

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

OpenNest.Core/PushDirection.cs

@@ -0,0 +1,11 @@
+
+namespace OpenNest
+{
+    public enum PushDirection
+    {
+        Up,
+        Down,
+        Left,
+        Right
+    }
+}