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/AccumulatingProgress.cs

@@ -26,7 +26,6 @@ namespace OpenNest.Engine.Fill
                 combined.AddRange(previousParts);
                 combined.AddRange(value.BestParts);
                 value.BestParts = combined;
-                value.BestPartCount = combined.Count;
             }
 
             inner.Report(value);