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/PairFiller.cs

@@ -88,8 +88,14 @@ namespace OpenNest.Engine.Fill
                         sinceImproved++;
                     }
 
-                    NestEngineBase.ReportProgress(progress, NestPhase.Pairs, plateNumber, best, workArea,
-                        $"Pairs: {i + 1}/{candidates.Count} candidates, best = {best?.Count ?? 0} parts");
+                    NestEngineBase.ReportProgress(progress, new ProgressReport
+                    {
+                        Phase = NestPhase.Pairs,
+                        PlateNumber = plateNumber,
+                        Parts = best,
+                        WorkArea = workArea,
+                        Description = $"Pairs: {i + 1}/{candidates.Count} candidates, best = {best?.Count ?? 0} parts",
+                    });
 
                     if (i + 1 >= EarlyExitMinTried && sinceImproved >= EarlyExitStaleLimit)
                     {