fix(ui): show live per-angle/per-candidate detail during nesting

Don't overwrite the Detail label with phase-level reports — let the
per-angle and per-candidate descriptions from the parallel loops remain
visible. Only clear the label on completion.

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

OpenNest.Engine/NestEngine.cs

@@ -866,7 +866,7 @@ namespace OpenNest
                 BestDensity = score.Density,
                 UsableRemnantArea = workArea.Area() - totalPartArea,
                 BestParts = clonedParts,
-                Description = $"{phase}: {score.Count} parts, {score.Density:P1}"
+                Description = null
             });
         }
 

OpenNest/Forms/NestProgressForm.cs

@@ -38,9 +38,8 @@ namespace OpenNest.Forms
             densityValue.Text = progress.BestDensity.ToString("P1");
             remnantValue.Text = $"{progress.UsableRemnantArea:F1} sq in";
 
-            descriptionValue.Text = !string.IsNullOrEmpty(progress.Description)
-                ? progress.Description
-                : "\u2014";
+            if (!string.IsNullOrEmpty(progress.Description))
+                descriptionValue.Text = progress.Description;
         }
 
         public void ShowCompleted()
@@ -53,6 +52,7 @@ namespace OpenNest.Forms
             UpdateElapsed();
 
             phaseValue.Text = "Done";
+            descriptionValue.Text = "\u2014";
             stopButton.Text = "Close";
             stopButton.Enabled = true;
             stopButton.Click -= StopButton_Click;