refactor: simplify NestProgress with computed properties and ProgressReport struct

Replace stored property setters (BestPartCount, BestDensity, NestedWidth,
NestedLength, NestedArea) with computed properties that derive values from
BestParts, with a lazy cache invalidated on setter. Add internal
ProgressReport struct to replace the 7-parameter ReportProgress signature.
Update all 13 callsites and AccumulatingProgress. Delete FormatPhaseName
in favor of NestPhase.ShortName() extension.

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

OpenNest.Tests/AccumulatingProgressTests.cs

@@ -18,7 +18,7 @@ public class AccumulatingProgressTests
         var accumulating = new AccumulatingProgress(inner, previous);
 
         var newParts = new List<Part> { TestHelpers.MakePartAt(20, 0, 10) };
-        accumulating.Report(new NestProgress { BestParts = newParts, BestPartCount = 1 });
+        accumulating.Report(new NestProgress { BestParts = newParts });
 
         Assert.NotNull(inner.Last);
         Assert.Equal(2, inner.Last.BestParts.Count);
@@ -32,7 +32,7 @@ public class AccumulatingProgressTests
         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 });
+        accumulating.Report(new NestProgress { BestParts = newParts });
 
         Assert.NotNull(inner.Last);
         Assert.Single(inner.Last.BestParts);

OpenNest.Tests/NestProgressTests.cs

@@ -0,0 +1,100 @@
+using OpenNest.Geometry;
+
+namespace OpenNest.Tests;
+
+public class NestProgressTests
+{
+    [Fact]
+    public void BestPartCount_NullParts_ReturnsZero()
+    {
+        var progress = new NestProgress { BestParts = null };
+        Assert.Equal(0, progress.BestPartCount);
+    }
+
+    [Fact]
+    public void BestPartCount_ReturnsBestPartsCount()
+    {
+        var parts = new List<Part>
+        {
+            TestHelpers.MakePartAt(0, 0, 5),
+            TestHelpers.MakePartAt(10, 0, 5),
+        };
+        var progress = new NestProgress { BestParts = parts };
+        Assert.Equal(2, progress.BestPartCount);
+    }
+
+    [Fact]
+    public void BestDensity_NullParts_ReturnsZero()
+    {
+        var progress = new NestProgress { BestParts = null };
+        Assert.Equal(0, progress.BestDensity);
+    }
+
+    [Fact]
+    public void BestDensity_MatchesFillScoreFormula()
+    {
+        var parts = new List<Part>
+        {
+            TestHelpers.MakePartAt(0, 0, 5),
+            TestHelpers.MakePartAt(5, 0, 5),
+        };
+        var workArea = new Box(0, 0, 100, 100);
+        var progress = new NestProgress { BestParts = parts, ActiveWorkArea = workArea };
+        Assert.Equal(1.0, progress.BestDensity, precision: 4);
+    }
+
+    [Fact]
+    public void NestedWidth_ReturnsPartsSpan()
+    {
+        var parts = new List<Part>
+        {
+            TestHelpers.MakePartAt(0, 0, 5),
+            TestHelpers.MakePartAt(10, 0, 5),
+        };
+        var progress = new NestProgress { BestParts = parts };
+        Assert.Equal(15, progress.NestedWidth, precision: 4);
+    }
+
+    [Fact]
+    public void NestedLength_ReturnsPartsSpan()
+    {
+        var parts = new List<Part>
+        {
+            TestHelpers.MakePartAt(0, 0, 5),
+            TestHelpers.MakePartAt(0, 10, 5),
+        };
+        var progress = new NestProgress { BestParts = parts };
+        Assert.Equal(15, progress.NestedLength, precision: 4);
+    }
+
+    [Fact]
+    public void NestedArea_ReturnsSumOfPartAreas()
+    {
+        var parts = new List<Part>
+        {
+            TestHelpers.MakePartAt(0, 0, 5),
+            TestHelpers.MakePartAt(10, 0, 5),
+        };
+        var progress = new NestProgress { BestParts = parts };
+        Assert.Equal(50, progress.NestedArea, precision: 4);
+    }
+
+    [Fact]
+    public void SettingBestParts_InvalidatesCache()
+    {
+        var parts1 = new List<Part> { TestHelpers.MakePartAt(0, 0, 5) };
+        var parts2 = new List<Part>
+        {
+            TestHelpers.MakePartAt(0, 0, 5),
+            TestHelpers.MakePartAt(10, 0, 5),
+        };
+
+        var progress = new NestProgress { BestParts = parts1 };
+        Assert.Equal(1, progress.BestPartCount);
+        Assert.Equal(25, progress.NestedArea, precision: 4);
+
+        progress.BestParts = parts2;
+        Assert.Equal(2, progress.BestPartCount);
+        Assert.Equal(50, progress.NestedArea, precision: 4);
+    }
+}