refactor(engine): extract AccumulatingProgress from StripNestEngine

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

OpenNest.Tests/AccumulatingProgressTests.cs

@@ -0,0 +1,51 @@
+namespace OpenNest.Tests;
+
+public class AccumulatingProgressTests
+{
+    private class CapturingProgress : IProgress<NestProgress>
+    {
+        public NestProgress Last { get; private set; }
+        public void Report(NestProgress value) => Last = value;
+    }
+
+    [Fact]
+    public void Report_PrependsPreviousParts()
+    {
+        var inner = new CapturingProgress();
+        var previous = new List<Part> { TestHelpers.MakePartAt(0, 0, 10) };
+        var accumulating = new AccumulatingProgress(inner, previous);
+
+        var newParts = new List<Part> { TestHelpers.MakePartAt(20, 0, 10) };
+        accumulating.Report(new NestProgress { BestParts = newParts, BestPartCount = 1 });
+
+        Assert.NotNull(inner.Last);
+        Assert.Equal(2, inner.Last.BestParts.Count);
+        Assert.Equal(2, inner.Last.BestPartCount);
+    }
+
+    [Fact]
+    public void Report_NoPreviousParts_PassesThrough()
+    {
+        var inner = new CapturingProgress();
+        var accumulating = new AccumulatingProgress(inner, new List<Part>());
+
+        var newParts = new List<Part> { TestHelpers.MakePartAt(0, 0, 10) };
+        accumulating.Report(new NestProgress { BestParts = newParts, BestPartCount = 1 });
+
+        Assert.NotNull(inner.Last);
+        Assert.Single(inner.Last.BestParts);
+    }
+
+    [Fact]
+    public void Report_NullBestParts_PassesThrough()
+    {
+        var inner = new CapturingProgress();
+        var previous = new List<Part> { TestHelpers.MakePartAt(0, 0, 10) };
+        var accumulating = new AccumulatingProgress(inner, previous);
+
+        accumulating.Report(new NestProgress { BestParts = null });
+
+        Assert.NotNull(inner.Last);
+        Assert.Null(inner.Last.BestParts);
+    }
+}