feat(engine): add NestPhase enum and NestProgress data model

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

OpenNest.Engine/NestProgress.cs

@@ -0,0 +1,22 @@
+using System.Collections.Generic;
+
+namespace OpenNest
+{
+    public enum NestPhase
+    {
+        Linear,
+        RectBestFit,
+        Pairs,
+        Remainder
+    }
+
+    public class NestProgress
+    {
+        public NestPhase Phase { get; set; }
+        public int PlateNumber { get; set; }
+        public int BestPartCount { get; set; }
+        public double BestDensity { get; set; }
+        public double UsableRemnantArea { get; set; }
+        public List<Part> BestParts { get; set; }
+    }
+}