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
             });
         }