fix(ui): set Description in phase-level progress reports

ReportProgress was not setting Description, so the Detail row always
showed the default em-dash. Now each phase report includes a meaningful
description, and UpdateProgress always updates the label (resetting to
em-dash when null).

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

OpenNest.Engine/NestEngine.cs

@@ -865,7 +865,8 @@ namespace OpenNest
                 BestPartCount = score.Count,
                 BestDensity = score.Density,
                 UsableRemnantArea = workArea.Area() - totalPartArea,
-                BestParts = clonedParts
+                BestParts = clonedParts,
+                Description = $"{phase}: {score.Count} parts, {score.Density:P1}"
             });
         }
 

OpenNest/Forms/NestProgressForm.cs

@@ -38,8 +38,9 @@ namespace OpenNest.Forms
             densityValue.Text = progress.BestDensity.ToString("P1");
             remnantValue.Text = $"{progress.UsableRemnantArea:F1} sq in";
 
-            if (!string.IsNullOrEmpty(progress.Description))
+            descriptionValue.Text = !string.IsNullOrEmpty(progress.Description)
-                descriptionValue.Text = progress.Description;
+                ? progress.Description
+                : "\u2014";
         }
 
         public void ShowCompleted()