feat: add IPartSequencer interface and SequencedPart

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

OpenNest.Engine/Sequencing/IPartSequencer.cs

@@ -0,0 +1,14 @@
+using System.Collections.Generic;
+
+namespace OpenNest.Engine.Sequencing
+{
+    public readonly struct SequencedPart
+    {
+        public Part Part { get; init; }
+    }
+
+    public interface IPartSequencer
+    {
+        List<SequencedPart> Sequence(IReadOnlyList<Part> parts, Plate plate);
+    }
+}