refactor(engine): extract AccumulatingProgress from StripNestEngine

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

OpenNest.Engine/AccumulatingProgress.cs

@@ -0,0 +1,35 @@
+using System;
+using System.Collections.Generic;
+
+namespace OpenNest
+{
+    /// <summary>
+    /// Wraps an IProgress to prepend previously placed parts to each report,
+    /// so the UI shows the full picture during incremental fills.
+    /// </summary>
+    internal class AccumulatingProgress : IProgress<NestProgress>
+    {
+        private readonly IProgress<NestProgress> inner;
+        private readonly List<Part> previousParts;
+
+        public AccumulatingProgress(IProgress<NestProgress> inner, List<Part> previousParts)
+        {
+            this.inner = inner;
+            this.previousParts = previousParts;
+        }
+
+        public void Report(NestProgress value)
+        {
+            if (value.BestParts != null && previousParts.Count > 0)
+            {
+                var combined = new List<Part>(previousParts.Count + value.BestParts.Count);
+                combined.AddRange(previousParts);
+                combined.AddRange(value.BestParts);
+                value.BestParts = combined;
+                value.BestPartCount = combined.Count;
+            }
+
+            inner.Report(value);
+        }
+    }
+}