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.Engine/Fill/ShrinkFiller.cs

@@ -79,8 +79,14 @@ namespace OpenNest.Engine.Fill
 
             var desc = $"Shrink {axis}: {bestParts.Count} parts, dim={dim:F1}";
 
-            NestEngineBase.ReportProgress(progress, NestPhase.Custom, plateNumber,
-                allParts, workArea, desc);
+            NestEngineBase.ReportProgress(progress, new ProgressReport
+            {
+                Phase = NestPhase.Custom,
+                PlateNumber = plateNumber,
+                Parts = allParts,
+                WorkArea = workArea,
+                Description = desc,
+            });
         }
 
         /// <summary>