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/Nfp/AutoNester.cs

@@ -74,8 +74,15 @@ namespace OpenNest.Engine.Nfp
 
             Debug.WriteLine($"[AutoNest] Result: {parts.Count} parts placed, {result.Iterations} SA iterations");
 
-            NestEngineBase.ReportProgress(progress, NestPhase.Nfp, 0, parts, workArea,
-                $"NFP: {parts.Count} parts, {result.Iterations} iterations", isOverallBest: true);
+            NestEngineBase.ReportProgress(progress, new ProgressReport
+            {
+                Phase = NestPhase.Nfp,
+                PlateNumber = 0,
+                Parts = parts,
+                WorkArea = workArea,
+                Description = $"NFP: {parts.Count} parts, {result.Iterations} iterations",
+                IsOverallBest = true,
+            });
 
             return parts;
         }

OpenNest.Engine/Nfp/SimulatedAnnealing.cs

@@ -277,8 +277,15 @@ namespace OpenNest.Engine.Nfp
         private static void ReportBest(IProgress<NestProgress> progress, List<Part> parts,
             Box workArea, string description)
         {
-            NestEngineBase.ReportProgress(progress, NestPhase.Nfp, 0, parts, workArea,
-                description, isOverallBest: true);
+            NestEngineBase.ReportProgress(progress, new ProgressReport
+            {
+                Phase = NestPhase.Nfp,
+                PlateNumber = 0,
+                Parts = parts,
+                WorkArea = workArea,
+                Description = description,
+                IsOverallBest = true,
+            });
         }
     }
 }